'Physical Resistant' monsters not effected by 100% physical damage reduction ignore

I ran into a set of physical resistant monsters with no other defensive mods and could not hurt them with Brutality on (even the Magic rarity ones were almost invulnerable to physical damage). This happened during Ritual on a map with World Eater activated, and everything else on the map was easy to kill including the area boss. The character had 100% chance for hits to ignore enemy physical damage reduction (Physical Mastery + Bastion Breaker) on a hit only build.

Is physical resistance different from physical damage reduction? And if so, how do you reduce it? Overwhelm enemy physical damage reduction was removed from the game, but it is worded similarly, so I am not sure what if anything applies to physical resistance. Or is this a bug?

I experienced the same thing - it was very frustrating.