The Pre-Sequel! Enemies resistant against everything?!

Hello guys. Im playing TPS on PS4 and for about 3 days, sometimes if I shoot on any kind of enemy (Scav, Craggon, Lost Legion etc.) the message resistant is popping up and is always coming back. This is at any kind of damage (corrosion, shock, fire, normal, explosive, even melee damage; I didn´t try cryo yet). So I even can hit them with criticals but I cant say if there is less damage now or not. I cant understand.
I also do not have a special shield, class-mod or oz-kid, and the weapon I use doesn´t matter, too.
Is this a bug? Or am I just too stupid to understand the game correctly? And how to fix that? Thanks for any help.