I can answer this within the context of Romania's Legionary Movement (Iron Guard), as it figured into the thesis I just completed about them. The short answer is: yes, this did disillusion them, but they were disillusioned with the Nazis specifically and not with fascism in general.

I'll start with some context:

The Legionary Movement in Romania was founded in 1927 as an explicitly antisemitic, anti-communist organization, which (like many other fascist organizations) saw the two as being connected. It was founded independently of the German or Italian fascist movements and took on quite different characteristics (it was heavily Orthodox Christian, very traditional, and wasn't necessarily interested in collaborating with other ultra-nationalist movements* as it saw itself as unique to the Romanian situation).

After WWI, Romania had gained a huge amount of territory that it considered part of "Greater Romania": Transylvania, Bukovina, the Banat... and they considered these areas rightfully theirs. The Molotov-Ribbentrop Pact was devastating for Romanians as it meant that some of this territory was then annexed to the Soviet Union.

I should note that at the time of the Pact, Romania was not led by the Legionary Movement — a couple of their members were briefly in the leading Gheorghe Tătărescu cabinet (specifically Horia Sima, who was secretary of education for something like ~3 days), but they were not in power. However, the outrage caused by this fresh loss of territory caused then-leader Tătărescu to step down. He was replaced by Ion Gigurtu, a far-right politician. However, Romania experienced further loss of territory due to the German/Italian-led Second Vienna Award, which caused chunks of Transylvania to be transferred to Hungary. This again outraged Romanians, and following huge protests, the fascist Legionary Movement came to power in September 1940 (1).

So, one thing useful to note here is that Romania's fascists came to power specifically as a protest against politicians complying with Nazi Germany — these losses of territory, although transferred to Hungary and the Soviet Union, were both the result of Nazi-led pacts, which local Romanians saw as an attack and a tearing-apart of their country.

With that context, how did the Legionary Movement actually react? Were they disillusioned?

As I said earlier, the short answer is yes, but it might be slightly more complicated due to later events. The Legionary Movement came to power after outrage at Nazi policies affecting Romania, but they did attempt to mend relations and collaborate more directly with the Nazis, which until now they hadn't done. Horia Sima, the leader of the Legionary Movement, was also in a peculiar situation: he was not actually the leader of the country, but the Vice-President, as experienced military general Ion Antonescu had been brought in to co-lead with them. Antonescu and Sima did not get along, to say the least, and this relationship was extremely tense: in issues of foreign and domestic policy they fought each other endlessly. One way this conflict manifested was in a race to (perhaps ironically) befriend Nazi Germany. To cut a long story short, the Germans did not trust the Legionary Movement, basically seeing them as just inexperienced terrorists, and during a meeting with Hitler (which Sima refused to attend), it was recommended to Antonescu to just cease relations with the Legionary Movement (2).

Following this meeting, Antonescu denounced the Legionary Movement on the recommendation of Hitler. After only about 4.5 months in power, in January 1941 the Legionary Movement staged a coup against Antonescu. Besides the massive and horrific anti-Jewish pogroms that this caused, as well as the multiple days of fighting and mass arrests, it marked a very important turning point in the Legionary Movement's history: they were now illegal in Romania and were forced to flee. Because of a couple of sympathetic German officials, Sima escaped to Germany (alongside some others). However, upon arriving in Germany, the Romanian fascists found themselves under house arrest, and then when relations further deteriorated (Sima was not a very happy prisoner and constantly escaped) they were moved to a series of concentration camps. They were only released once it was clear that the Soviet Union had more influence than Antonescu in Romania, and the Germans prompted Sima to form a Romanian government-in-exile for a very brief period until the end of the war. At this point, despite previous interactions, he collaborated willingly (3).

So it's safe to say that the Legionary Movement was not very happy with the Germans for a number of reasons: they had sold off Romanian territory twice (which they saw as not only anti-Romanian but pro-communist) and then to make things worse they had imprisoned fellow fascists in concentration camps. With that being said, the Legionary Movement certainly did attempt (if somewhat half-heartedly/briefly) to form relations with Germany in 1940, but as soon as it was clear that Antonescu would win out, they had no chance. However, they never became disillusioned with fascism and in fact remained firm fascists even after the war.

Sima himself would become a somewhat prolific publisher and would weigh in on both contemporary and historical events. He was not shy to address what his feelings were towards Nazi Germany! He wrote an entire memoir, Prizonieri ai Puterilor Axei ("Prisoners of the Axis Powers") dealing with his imprisonment by Nazi Germany and denouncing the entire operation. In his 1951 Destinée du nationalisme, where he contemplated what ultra-nationalism would look like in a post-WWII world, he tore the Nazis apart: he accused them (and the Italians) of "abandoning antibolshevism", adopting "obsolete ideological stands", not understanding "the difference between 'national' and 'nationalism'", being imperialist and illegally occupying friendly states, and even simply not understanding what fascism was in the first place (4). By the 1960s Sima would use "Hitlerian" pejoratively, for example when describing the trial of Adolf Eichmann, which he saw as the Israeli adoption of "Hitlerian" and "Bolshevik" practices against international law (Sima often treads the line between Nazi sympathizing and denouncing of Nazism quite closely, as you can see...) (5)

So, I hope that sort of begins to answer your question (?). The Legionary Movement certainly did not become disillusioned with fascism as an ideology, but with the Nazis as a group that they saw as predatory and misunderstanding of fascism/ultra-nationalism. That being said, the answer isn't necessarily so simple: the Legionary Movement did willingly collaborate with the Nazis on multiple occasions (attempting to court them in 1940, working for them as a Romanian government-in-exile, etc.) and they were just as virulently antisemitic, ultra-nationalist, anticommunist, and anti-foreign as the Nazis (if not even moreso). Horia Sima's own opinion of the Nazis was not only influenced by events like the Molotov-Ribbentrop Pact, but also his own imprisonment by the Nazis, which certainly influenced his opinion of them. Ultimately, though, local dissatisfaction with Nazi policy which led to loss of territory twice in Romania was what brought the Legionary Movement to power, and this influenced their stance towards the Germans until the end of the war.

Footnotes:

1. Roland Clark, Holy Legionary Youth: Fascist Activism in Interwar Romania (Cornell University Press), 220-223.

2. Ilarion Tiu, The Legionary Movement after Corneliu Codreanu: From the Dictatorship of King Carol II to the Communist Regime (February 1938 - August 1944) (East European Monographs), 179-180.

3. See Horia Sima's Prizonieri ai Puterilor Axei, which is his memoir dealing with this period, or Ilarion Tiu from about p. 200 onwards.

4. See Horia Sima, Destinée du nationalisme (Paris: PEG, 1951).

5. Horia Sima, Articole Politice (Colectia Omul Nou), 105-109.

*Note: they had collaborated with other ultra-nationalist orgs by way of the Spanish Civil War, which they sent some members to fight in, and they made friends with a few individual Italians (notably Julius Evola) but they had not made great efforts to befriend the Germans.

edits to fix one run-on sentence