Yeah, I reported the same issue on testlive (see link below for additional details). A version of it has been going on for a long time, but it used to still be possible to strike them, because they only sank part way. Now they sink too far and cannot be struck, which makes getting into the Dregs far more complicated. Hopefully this gets looked at at some point.
Workaround options until it gets fixed - backing up more or less to the wall of the chamber can get them to pop back up, but that can wind up with a lot of messing about timing trying to hit them on the lid as they sink back in each time you go back on. Most reliable option is to knockout one of the darfari and drag them onto the lid with a rope, then murder them (so long as they don’t also sink in…)
Best thing to do for this stupid mechanic and it always being broken in someway is just knock out an npc close to the end, kill the rest and drag that npc close to the entrance and kill it, saves a major headache of them falling thru or getting stuck and immune in the entrance…….can’t believe this is one of the first dungeons new players will do and it’s always like this in some way.
I’d be happy if they just opened the entrance and left it open for good……seriously