But as Alara said, it informs in bulk.
I like to use a specific episode of Supernatural as an example. They shot the show in Canada, but this episode allegedly took place just outside of New Orleans.
There was one named Black character, who was found dead. There was one singular black extra in the background, existing for maybe 5 to 10 seconds.
Everyone else was white, even the nonspeaking extras.
That's incredibly... Not Louisiana as a whole, let alone the New Orleans area in particular. But this one episode was just the one episode, right?
Except that there are hundreds of episodes of various shows filmed in very white places with little to no Black representation, and claiming to take place within the New Orleans area.
If you saw all of those, you'd subconsciously, maybe, wonder if New Orleans wasn't as Black as you thought, or if perhaps there was a lot of White gentrification going on (which, there is, but there's still a ton of Black people just, you know... Existing) - especially if you don't know the reality behind filming locations and such.
When we consume media for learning's sake - from the news, textbooks, etc. - we are looking for inconsistencies and lies. We have our bullshit filter up.
Buuuuut when we consume media for entertainment, that bullshit filter is borderline nonexistent! So if you see the same message repeated over and over and over again, it might begin to inform some of your subconscious biases.
There are certain things the brain looks for to differentiate between the two, as well, which has been studied at length. A lot of News Organizations use shady tactics to trick your brain into relaxing into Entertainment Mode, like having Loud Correspondants, emotional stories that feel like fiction, et cetera. Fox News is infamous for it, and it's why so many Fox viewers fall for shit - because they are being tricked into lowering that Bullshit Filter. Ancient Aliens does this kind of thing too... A lot of documentaries do. Super Size Me is one. It's... Bad.
Anyway. One episode or one movie or one book or one fanfic featuring a problematic trope isn't going to change the world.
But if it's one out of thousands, or even millions, that's when we, as writers and artists, owe it to ourselves to stop and ponder where that trope came from and why we're leaning on it so much, and if maybe we have some biases to work through.
We also owe it to the marginalized people in our lives to present them better to the world. Fanfic starts trends in writing, and I've been doing fanfic for 30 years so I can say that with relative certainty. If making something popular in fanfic happens, it starts to pop up in books, TV shows, etc.
So no, like. One instance isn't gonna be a huge issue.
It's if your instance is one of thousands or millions. And really, you owe yourself and those around you better than that. Not from a writing standpoint, go ahead and publish whatever (personally, I like to try and make the text point out, in some way, that what happened was Not Okay, even if it's just an idle thought a character has. Just so the cycle of bullshit gets interrupted a little).
But from a growing up and doing better standpoint.
You deserve it. As a treat.