Looks like its caused by the Divinity Unleashed mod and Pet Power interacting somehow. The HP gets a boost from DU, but with Pet Power on as well the HP drops instead. From what I can tell other summons are not affected.
I tried verifying the game and deleting the contents of the level cache folder but nothing changed. It was working fine up until today, and the DU mod has not been updated, nor has the game from what I can see. I have no idea why something would happen out of the blue like this with apparently nothing to trigger it.
