Enemies becoming randomly invulnerable.

I recently noticed that when I was fighting the Colossus in the Valley of the Titans, he just stopped taking damage. Nothing I threw at him hurt him at all, or if damage did get through, it was on the order of 2-3 hit points. This happened 2 or 3 times, an I have no idea why. I was playing a Gemling with the +10% Gem Quality ability. I have noticed a lot more enemies seeming to take a lot longer to kill than they should as well.

Seen this many times, including jamara boss, which kinda hurts. Just run a way for a bit and they all of a sudden start taking damage again. Sometimes it takes quite some time tho. I have this issue very often when doing excavations, and my minions kan hit them for ever until i run away for a bit. A bug for sure.