Hard to describe this one without a minor spoiler...

If you tell Mayrina about her family at the Riverside Teahouse, you fight a Hag for one round before the hag flees. Problem is, if you kill the hag in that first round (which is pretty easy to do with a boosted sleep spell) it leads to a bug regarding the hags enthralled servants. You see, normally when the hag successfully flees, you face her again later and if you choose to kill her, you can avoid killing her servants and even free one of them. But if you kill the hag in that first round right after telling Mayrina about her family, the servants become bugged. They stay red, but don't attack the party on sight. However, being red, you can't talk to them, which is how you normally free the one wearing the Mask of Regret. This bug has been around since early Early Access (I think I reported it about a year ago), so I'm a bit surprised it is still in the final release.