All those missing hp sound like you lost some buff from either flanking, the priests’ buff (1087 to 15xx depending on build) or a demonologist’s pet, or several of them.
Dead or unbuffed people in your group can maybe explain why you have been hit more too : maybe you didn’t have a mage shield any more.
The only bug I have heard/seen with T6 armour is having the wrong number of stacks compared to how many pieces you have, which gets fixed by removing all your pieces are re-equipping them. This having less hp sounds very weird.
Next time, try to resurrect outside, unequip everything, zone in and equip your gear again.
There is an old bug that seems to make your gear stats not fully apply sometimes.