So, firstly, it doesn't appear that you fully understand the criticism of Stalinism from a modern leftist standpoint: "communism" is a vast and often contradictory set of beliefs based primarily on the abolition of private property, the removal of coercive hierarchies through primarily the deconstruction of the state, and the democratic distribution and ownership of the means of production.

Soviet communism never achieved that vision. Some would argue that it never even fairly tried to achieve it. Soviet communism was built on the notion of "the dictatorship of the proletariat," which was envisioned as a median stage of centrally-directed action designed at using the mechanisms of state to destroy itself. As the term implies, the "dictatorship" was intended - in theory - to be highly democratic and to put the needs of the proletariat first, but could utilize the advantages of control and force already in place with the state to promote social and economic leveling, with the understanding that, prior to a revolution and during it, those with power are unlikely to peacefully surrender it - hence the need for a revolution in the first place.

The term itself was actually coined by Joseph Weydemeyer (who founded the first Marxist organization on the United States and eventually served as a colonel in the Union army during the American Civil War), not by Karl Marx, though Marx later adopted it and wrote of the principle in later works.

Anyway the point is there is some basis for the belief that Stalinism represented in fact a portion of Marxist/communist ideology. But that didn't change the fact that the Soviet Union was immediately criticized by a good number of communists and other leftists.

One of the first was Rosa Luxemburg in 1918. She sharply condemned Lenin "and his comrades" for dissolving the Constituent Assembly - an elected group of representatives from around Russia tasked with creating a constitution - and moving the Bolshevik government from something that represented itself as provisional to something more permanent. Luxemburg saw this as the action of a bourgeois class subverting popular will to their own ends:

To be sure, in all these cases, it was really not the "people" who engaged in these reactionary policies, but only the bourgeois and petit bourgeois classes, who - in sharpest opposition to their own proletarian masses - perverted the "national right of self-determination" into an instrument of their counter-revolutionary class policies.

The Constituent Assembly was hardly perfect, and some have even argued that it was never adequate to represent the will of the people, but Luxemburg's criticisms were only the first of many. The anarchist Emma Goldman wrote a book in 1923 called My Disillusionment in Russia, describing the lead-up to, and the crushing of, the Kronstadt Rebellion.

Later, and more popularly, George Orwell's Animal Farm was explicitly written as a critique of Stalinism, not, as is so often represented today, as a critique of communism or socialism in general. Orwell had fought in the Spanish Civil War, and many of his own experiences were based on the behavior of certain factions on the republican side of that conflict.

In short, "communists" have criticised the Soviet Union from the very beginning. Luxemburg, Goldman, and Orwell are not the only people to have done it, and to find that modern self-avowed communists would make an effort to point out the Soviet Union's failures is not at all novel.

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You can read Luxemburg's entire pamphlet here, there's far more to her critique than the tiny blurb I posted.

Goldman's work is available here and here, split into two parts because of a publishing issue in the United States.

Orwell wrote of his experiences in the Spanish Civil War in Homage to Catalonia