I don't believe that it's limited to 40 HP gain. I didn't do a full rest in the underdark, on the second battle I had it down to 49 HP, when it came back for the third battle it was full health again; similar for the 4th it was down below half before burrowing.
It's the law of unintended consequences played out on screen, coupled with bad modifications to an already 5e balanced foe. There was no need to bump the Bulette's AC from 14 to 17, as it already has 94 HP, or give it an acid AOE attack. If Larain was concerned that it would be too easy with AOE/surface spam and barralmancy, then they should cut those DOS features out rather than butcher 5e to try to make a poorly designed encounter work.
Even if we go with the "you'll be higher level in the release game" you still are going to be hard pressed to survive this fight given that it can push you off a high spot and then belly flop you for near 250 damage; cheesing it shouldn't be the most viable option.
My 5 battles were as follows, 2 x minotaur area, 1x altar area just beyond the hook horror battle, 1x at the small chasm leap near the myconid entrance, 1x at the top of the climbing wall opposite the path to the tower.
I don't know the exact dice that can be regained, as it doesn't say in the ability description, but I'm guessing 4d6, 3d8 or 2d12. The highest value I've seen is 24 per burrow.
I just ran a similar test as my previous vid where had it down to 8. Except after it burrowed, I slaughtered the Myconid colony, levelled my characters up to 4, short rested twice, and triggered it from the location by the Sussur tree. It only regained 32 HP.