You covered all the pertinent bases here for sure. I think what a lot of people call FWB is just straight up hooking up on their part and maybe perceived as friends with benefits on the other person’s part, and that can cause an issue from the beginning.
Years back, I had an acquaintance who I would meet once a month or so, literally for a booty call. We didn’t hang out otherwise, we rarely texted between hookups, it was clear what was going on.
At least for me.
After a few months of this, I ended up dating someone fairly exclusively and no longer had need of the booty call. He got seriously upset. In his mind, we were much more than just a hook up, and I honestly don’t know how. If we were dating, and that’s how he thought a relationship went, I feel sorry for anyone he was ever with.