Then it's broken in the patch, because I've faced it 4 times now, done plenty of damage to it, with only short rests, and by the time it comes back it has full health again.. As to your solution, gaming a broken system being the most effective way to beat an enemy, means that the encounter and/or enemy design is broken, and it's the complete antithesis of DnD.
I've been investigating a bug where someone claims that they killed multiple bulettes, because I want to collect them, so I've killed one at least 15 times this patch. Most of those times letting it burrow on purpose. It regains ~20 hp when it burrows in and another ~20 hp when it burrows out, if you don't rest.
That being said, what I will concede, is that, if you are invisible or out of range when it comes up after being burrowed, it will auto heal for full health within a couple seconds.
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.
