OK, I've now tracked down one cause of the corrupted NPC data - at least in the Blushing Mermaid. It's fully reproducible. It happens when:

- You have non-lethal damage on
- You knock out an enemy that has an effect that restores HP at the start of its turn - in this case, "Boon of Servitude"

I've sent Larian another - very detailed - bug report with this information, links to this thread, and to the many Reddit threads of people suffering from this bug. Hopefully it finally gets some attention!