I have a couple of corrections I'd like to make, though getting to the core of your issue is difficult.

When discussing the West Bank occupied by Israel, the fourth Geneva Convention is often cited - namely, that annexation after 1949 is illegal by international law

This is not actually the case. Annexation by the use of force was not made illegal in 1949. The Geneva Convention provision that's usually cited from 1949 with regards to Israel deals not with annexation, but with settlements. The provision, from Article 49(6) of the Fourth Geneva Convention, covers the transfer of population into occupied territory.

Without getting into the merits or debates on that, that's very different from the legal question of annexation and its legality. That is a thornier question, because it's not only a semi-part of the UN Charter (in Article 2(4), saying that all members must refrain from the threat or use of force). Other sources of law cited with regards to Israel are the introductory provisions of UN Security Council Resolutions, like 242, which stress the inadmissibility of the acquisition of territory by force. There are legal arguments about whether this means that Israel can annex no territory or some territory, but that's also getting into the weeds. There's also legal historical debates about when this norm came into being, and whether it applied only to the aggressive use of force, rather than defensive use of force.

None of this is very clear, so let's trace back to the original norm and law besides the UN Charter. War was "outlawed" in 1928, but conquest clearly continued after that. The norm itself seems to have only solidified more after the end of WWII, and the adoption of the UN Charter. Even then, the norm has notably been ignored in a wide variety of cases. In Tibet, for example, there was a treaty of questionable validity that ended the Chinese invasion with its incorporation into China. However, the opposition to it appears to have been less notable worldwide, and it has been viewed more as a fait accompli than a valid legal action. The lack of concrete norms or rules against the use of force against other states (and China did contest whether Tibet was a state at all) was likely a factor given the timeline; 1949 was very different from 1967, by which point groups like the International Law Commission and others were already solidifying treaties and proposals on the inadmissibility of acquiring territory by force.

So if you're asking the differences, I'll be honest, I haven't managed to dig many up factually that would help explain why they're treated differently. What I have managed to find is that the timelines were different, and many states chose to sort of say "fine" during the 1940s, but by the 1960s times had "changed" and they drew a line in the sand, so to speak. This hasn't stopped annexations from happening, or settlements from being placed, many of which have received no serious scrutiny. As one example, the UN Security Council has not, to my knowledge, condemned Turkey, Armenia, India/Pakistan, or any other state in the way they have Israel, by saying those settlements constitute a violation of international law and the Fourth Geneva Convention. Some of those cases arose after 1967, making the inconsistency even harder to explain (i.e. Turkey in the 1970s with Northern Cyprus). I wish I had a better answer for you, but perhaps the best one that illustrates the differences comes down to:

1. Timeline, where applicable, i.e. differentiating 1949 from 1967.

2. Attention, for whatever reasons some issues are given attention while others are not, and which are hard to explain. For example, the Soviets were closely aligned with the Arab world and drew much attention to Israel since that harmed Soviet allies (i.e. Arab states and populations), but they cared far less about Turkey because both sides were NATO members in that dispute.

3. Power dynamics, namely that it seems impossible to make a difference or pressure China when looking at Tibet vis a vis China, while pressuring a small state like Israel or fostering shifts in its superpower ally (the United States) to harm it or place pressure on it seems far easier.

4. UN dynamics, namely that every state receives the same vote, and anti-democratic states or anti-US states have long held significant sway in that body, and there are some 20+ Arab states with longstanding dislike for Israel but far fewer states who have a strong hatred for China that lets them throw diplomatic weight around.

Those are the only explanations I can think of, and historically there's not much real explanation for why one has been seen differently from another, at least not in the legal literature, besides "this is a fait accompli" type of language, or "it happened before the norm solidified", which explains Tibet but not the other cases like Turkey, Armenia, etc.. I'm not familiar with any other arguments.