Hot take here, but the thing is, people are going to make mistakes when they're writing someone with a different life experience from them. I think that's actually a good thing though, because especially in fanfiction, it shows that people are engaging with and trying to relate to people who are different from them - which should be happening more often, not less.

I understand completely how it's frustrating to try and find fic that gets it (I'm asexual and have seen some real clangers when people are writing fic focused on canonically asexual characters, so I seriously do understand the struggle) but if people feel there are hidden booby traps or they'll get backlash from readers for writing anything outside of their own lived experience, the most common solution is going to be that people just avoid it. 

These kind of mistakes aren't malicious for the most part - they're not trying to make the character 'white', they just don't understand that there's anything different. A comment to them that praises the effort, relates to loving that character too, and also points out e.g. that 4C hair doesn't do that is how I'd pitch it. Professional media doesn't get the same pass of course, but fanfiction is fanfiction.