I finally managed to start figuring out what made the hordes feel so much harder than before, and none of it has to do with the stagger system or tougher enemies.
There seem to be a lot more overlapping enemies now (i. e. hyperdensity), and the Beastmen seem particularly prone to it. That produces problems with any weapons that don’t cleave very well. The enemies also surround you more effectively now, making controlling hordes harder than before. That is partially tied to the enemies trying to push through the PCs much more than before, too, which adds to these. And finally, dodging needing more precision now, combined with more stuttering than before, makes avoiding hits and controlling hordes even more difficult.
All that, added with the backstab sound warning apparently being broken again and the Beastmen approaching very quietly combines to a pretty jarring experience. I do suggest (other than getting to fixing the found bugs as quickly as possible) reviewing the changes made to the slot system, which seems to be a significant culprit here.