oh my god, this is the most annoying thing ever, I know. It even seems that sometimes I do what PieceOfMind suggested and it doesn't work. There's SOMETHING I don't understand about the whole grouping thing, obviously. Maybe I was doing the trick wrong, I think a lot of the time, when I shift click on unit and it automatically selects a bunch, I'd try to "break" that group only with the separate group button. I guess next time I'll make sure I'm actually selecting the whole group. Still, if that's the problem, it doesn't make sense since the "separate group" button should separate any sticky group I have within a group... Anyway, it's certainly not intuitive.
Wow, I reread this and it's confusing. I guess that's why I never actually asked the question on CFC, I thought of asking but I wasn't able to word it.