Childe is probably the biggest offender from what I've seen in the Genshin fandom. People have hundreds of different ways to interpret that character and only a handful of them are close to canon accurate. It used to bug me a lot but now I just avoid any tag that has him in it. It's much easier on my sanity.

Throw Zhongli into the mix and the OOC tends to spread from Childe to him... Which is a shame because while they're not my OTP, I do think the two could work as a couple. But good luck finding a story featuring that pairing where both characters keep even a sliver of their canon personalities.

The other one that stands out was Tea/Anzu from Yugioh. Sweet character in the show, rabid mean girl in the fandom. It was absolutely insufferable.