IRL and online. It happens way too often.

A big part of it is that a lot of modern leftists were kind of forged in the post 9/11 culture and the reaction against Bush's forever wars, and they in turn inherited much of their political conscience from elder leftists who were forged in the Nixon years and the reaction against the Vietnam war.

So its become a kind of fait a complit on the left that being anti-war and anti American Imperialism is always good and necessary. And those are good positions to hold, but the problem is that people don't come to them through interrogative thought. They're just sort of handed to you as part of your welcome package. So when presented with a scenario where supporting the continuation of a military conflict - even when that support aligns you with the interests of US global influence - is actually a moral good, a lot of these people really struggle to break free of their own thought terminating cliches.

The Ukraine conflict simply does not fit into the same boxes that they're used to sorting ideas into, and because, by and large, most leftists detest war on a purely moral level (understandable, no one should be overjoyed at the notion of vast destruction of human life, though a depressingly large number of people are) they simply haven't spent enough time interrogating these ideas to have built a framework to handle complex cases like this.

Russia very carefully frames their propaganda to exploit these moral conflicts. NATO is a military alliance, ergo "NATO bad". America is an imperialist power, ergo anything that serves American interests is bad. Military conflicts lead to unacceptable loss of life, ergo anything which prolongs a conflict is bad. These are easy answers to complicated problems, so people naturally latch onto them.