Bosses in Maps and Asc immune to elemental Damage / no visual modifier

Good evening,

has anyone had the same bug? Running lightning merc and now had two rares in maps which became immune to my damage after a few hits.

They had no special modifier, no enhanced resists, no partial immunity. Weirdly enough they would sometimes get damaged when I would get really close and only shoot with galvanic shot. As soon as I went a bit further away the hits didnt connect anymore.

Has anyone else had that problem?

A friend of mine had the same issue with the boss in Ascendency 3, where after 40% the boss just stopped taking damage.