I know it’s in the known bugs but I have found something weird that might be the reason why the bosses are so tanky.
In difficultylevels.txt, the PlayerDamagePercentVSPrimeEvil is set to 10 for Nightmare and Hell.
In the current game, they are at 100.
In regular game(difficultylevels.txt)
Name PlayerDamagePercentVSPrimeEvil
Normal 100
Nightmare 100
Hell 100
In PTR 2.4 (difficultylevels.txt)
Name PlayerDamagePercentVSPrimeEvil
Normal 100
Nightmare 10
Hell 10