hollow basin
I just finished a coop Trial By Fire with someone where to my confusion, the skull icon indicated we went in on Survivor difficulty, but the difficulty was Apocalypse. I stayed with them to the end to be certain, the results screen clearly indicated Apocalypse both in name and EXP multiplier, and it was clear just from the combat that, it was Apocalypse.
I joined them from the Join Game feature with it set to search Boss Rush Apocalypse games (which was what my world was also set to at the time), and the skull icon changed back correctly to the Apocalypse icon once we returned to the Ward. I have joined someone else earlier in the same way where this icon stayed correct so it doesn't seem to happen every time. I haven't done a lot of coop recently, boss rush or otherwise, so I'm not even sure if this is specific to Boss Rush or not.
lavish perch
simple marsh