Okay, totally willing to believe this was user error, but I've been trying to properly roleplay my first full release playthrough and do what I think my character would even if I know it would be a bad idea.
This included putting on a whispering mask in Ethel's lair.
As expected ...
I lost control of my character to Ethel. However, I couldn't get it back again, and even when she saved against Ephemeral Whispers she remained a Hag's Pawn and I couldn't remove the mask. It's not possible to cast Protection from Evil/Good on a hostile creature so that wasn't an option.
And I tried going into turn-based mode to see if I was just not being quick enough, but then when I tried to end the turn of my Hag's Pawn character, it didn't work so I had to exit turn-based mode to continue.
I ended up having to reload in order to be able to progress. Or did I miss an obvious solution?