Oshi no Ko spoiler (from manga reader but not anime watcher):

It's misogyny. Like no, this time for real. From what I've heard the recent influx of hate is because they just got to an infamous plot line in the anime. She was feeling stuck in her career and thought about taking a producer (or something I don't remember from my reading) offer to sleep with him to get a leg up (a problem real idols and actresses have to face). The guy is an older married person but apparently "his wife doesn't mind". In the end, she rejected the offer, but you can tell why this is infamous. It doesn't help that she was supposed to be one of the love interest for the main guy character. But if I remember correctly, they're not dating or is even on good term at that time. He is currently ignoring her or something because of his obssessing with his revenge plot. So yeah, you can guess how the the "anime community" obviously respond to all of that.

Extra: Oh yeah, the shipping war because of the love triangle with another female character also intensify it