Yeah, I totally get the concern around that. Not in the context of a fanfic but in another writing collaboration, I wrote something for someone and they basically took bits and pieces from what I wrote and used it to say something different that wasn’t my original intention.
So there’s definitely reason to be concerned. I can see why being clear on what is and isn’t allowed when it comes to preferences. I’ve seen a few different fics on here from people who seem to be similar to what I write. In fact, I’ve been inspired by some other people’s writing to write my own version of the idea.
Maybe if there was some kind of approval process for collaborations where whoever creates the story has approval over what chapters do and don’t get posted.