Edit for clarity: I'm not asking why the Tankie/Anarchist grudge exist. I'm curious about what information sources - mentors, friends, books, TV, cultural osmosis, conveys that information to people. Where do individuals encounter this information and how does it become important to them. It's an anthropology question about a contemporary culture rather than a question about the history of leftism.
I've been thinking about this a bit lately. Newly minted Anarchists have to learn to hate Lenin and Stalin and whoever else they have a grudge against. They have to encounter some materials or teacher who teaches them "Yeah these guys, you have to hate these guys and it has to be super-personal like they kicked your dog. You have to be extremely angry about it and treat anyone who doesn't disavow them as though they're literally going to kill you."
Like there's some process of enculturation there, of being brought in to the culture of anarchism, and there's a process where anarchists learn this thing that all (most?) anarchists know and agree on.
Idk, just anthropology brain anthropologying. Cause like if someone or something didn't teach you this why would you care so much?
Most Westerners already hate communists and carry the grudge against the USSR. Anarchists don't really deviate too much from some generic Westerner. At best, they uphold people like John Brown or support Spanish syndicalists liquidating the clergy during the Spanish Civil War. But in terms of what faction/state/org to support, I honestly don't see a whole lot of difference between an anarchist and some progressive liberal. Their disagreement comes from tactics and ultimate goal, but they still support and oppose the same people in the end.
Seriously, what faction do anarchists support that someone who is a member of the US Green Party wouldn't support? Yes, anarchists think voting is cringe, but your average US progressive isn't going to oppose the Zapatistas. If you explain Kronstadt to your average US progressive, how many of them would support Trotsky crushing the rebellion with the Red Army over the striking sailors like the way anarchists do? As far as history is concerned, you could pretty much explain every single disagreement communists and anarchists have to a progressive liberal, and the progressive liberal will almost always side with the anarchist. I honestly can't think of a single instance where the progressive would go, "Okay, you anarkiddies are being cringe. The tankies have a point."
It's anecdotal but I have gotten liberals to understand that it was probably a bad idea when the Spanish Republicans started refusing guns/artillery from Stalin. Except I think most anarchists also agree that was probably a bad move.
If that counts
After thinking hard about this, the only thing I could think of that anarchists and communists would agree on is that the Zionist entity needs to be forcefully abolished while US progressives still think a two-state solution is tenable. Anarchists and communists at the very least critically support Hamas while US progressives still think Hamas beheaded imaginary babies.
There's also cases where all three groups can come to a consensus, and besides the obvious like opposing the NSDAP, white progressives, white anarchists, and white communists butt heads with Black radicals and Pan-africanists. For example, white progressives, white anarchists, and white communists do not support Marcus Garvey while Pan-africanists address him as "the Honorable Marcus Garvey." White progressives, white anarchists, and white communists oppose the Nation of Islam for being a reactionary Black supremacist organization while most Black radicals and Pan-Africanists critically support the NOI as a means in which Black people can be properly organized. In the same way that communists deviate from the status quo more than anarchists, Black radicals deviate from the status quo more than white radicals, and I would argue that their deviation is greater than communists.
Some of the most anti-anarchist sectarianism I've seen come from Pan-africanists and Black radicals who describe themselves as scientific socialists or Nkrumaist.