My raid team also encountered this bug.
It seemed to start with us as soon as we downed a boss on normal, then came back the next day with a slightly different roster. We got this message every time we tried to zone back in after a corpse run. For us to get back in, we would have to swap it to normal to be able to zone in, everyone logged out except raid lead, raid lead sets it to heroic, then everyone logs back in. We didn’t do the whole raid on the same night, so idk if that had anything to do with why it affected us.
It’s wasting a lot of time for raid teams trying to prog heroics.