80+ Map Bosses with "Extra" Energy Shield mod are Unkillable...

My damage is very very good. I kill lvl 82 rares in half a second.

If a lvl 80+ boss has an Energy Shield, it can take a full minute of uninterrupted damage to get it down, and then the boss does an ability or phase change that lets it reset.

This has to be a bug or unintended.