To return to the original topic, enemies definitely heal outside of combat now. I recently tried to weaponise Gale's necrotic death aura against the tomb raiders in the ruined temple of Jergal (by smashing in the roof to gather them all in the corridor and then letting Gale go down there and die in the middle of them) and to my disappointment, as soon as Gale was dead, combat was over, and his aura started damaging them... They started healing themselves up. And kept healing themselves. Unendingly.
There's a chance the bulette has similar ability to just heal itself right back up to full health outside of combat.
Something else is triggering the full heal if the numbers posted are correct. In this last playthrough, the bulette showed up 4 times and escaped with less than 50 HP in all but one of those instances, and in the 5th battle where I finally cheesed it and did enough damage in a single round to keep it from burrowing, it was back to full health at the start of the fight. That's with zero camps, and only the short rests. The only thing I can think of is that I did transition areas and go to the Kua-toa shrine.
You should load to a save before you fought the Bulette and try to repro it. It's not transition. I just did the Kua-Toa fight and back and it had normal burrow hp recovery. I also fast travelled to Overgrown Ruins, ran to Ethel's killed her, and went through the portal to Underdark and it had normal burrow hp recovery.
edit:Loading from an autosave while I was above ground and fast travelling back to underdark, It had a recovery of 52.
edit2: Yep, that's the bug. It's not using its full recovery ability. If you load a save you made outside of the main area of underdark, it gets an extra burrow.(I saw 8 to 60 and 8 to 67)
If you load that game, go to underdark, leave underdark, create a new save, load it, go back to underdark, it gets two extra burrows. (8 to 85 and 8 to 84)
