The thing about fandom is that there are a number of ways of looking at the experience:
What is common across several subfandoms in several forms of media
The subfandoms you don't associate with on specific forms of media
The subfandom you associate with across several forms of media
The subfandom you associate with on specific forms of media
Everything said has a context to it and not all of it needs to match the first option. Ultimately, fandom comes down to daily experience and if something becomes an echo chamber at a specific time, posts are made on them. Sometimes you don't associate with the fandom of a character you like and you make your peace doing that while associating with another fandom.
When that fandom starts to make you uncomfortable because of their digs at your other fave, it's just eating into your safe place. That's why people vent because they want to be able to do both. The person might still have the same criticisms of the overarching fandom and yet be tired that they can't enjoy both things in peace. It is the unavoidable nature of fandom regardless of what the object of interest is. No fandom is immune.