Monsters in delve gain up to 6 endurance charges - become immune to phys damage

Noticed this multiple times, always same scenario - rare monster with a mod "Endurance Charge when Hit" in delve with 6 endurance charges - it doesn't take phys damage (i'm playing phys dot). Can only kill it with dynamite.

It does obviously take damage before reaching 6 charges. I never saw 6 charges on monsters in delve before, so I don't know if this is the problem or the fact that the monster becomes immune to phys damage - I have crush on hit from awakened brutality support that should lower the phys reduction i guess.