Asking for a friend: He has multiple Daikin units and they're all doing the same thing, heating up to the set point, staying there for a couple of hours, and then stepping up by around 1 degree an hour until the room gets unbearably hot and they turn them off.  Here's a sample reading, it's similar in other rooms:

The horizontal red line is the set point, blue is the room temperature, and orange is the heat pump heating.

Daikin have looked into it and acknowledged there's a problem but things have kinda stalled after swapping out some of the controller boards.  Just wondering if anyone else is experiencing this.

Note that this appears to be a fault with the units, it's not anything to do with the environment or install, they were professionally and correctly installed and set up, and Daikin have been around to look and couldn't find any problems with the setup.