Attack Mastery Block-piercing not applying to Ultimatum

The Attack Mastery that reads Monsters cannot Block your Attacks does not appear to stop Ultimatum monsters from blocking your attacks if you pick the effect that gives them a 50% chance to block attack and spell damage.

Things were normal in the first round. I picked this effect round 2 and noticed monsters were not taking damage for seconds at a time of standing and attacking, watching their hp go down in short bursts. This lasted the rest of the encounter. I did not take Occasional Impotence.

I just had this, pretty annoying, please fix

Pretty sure this is still a thing. I think I'm also seeing it in Expedition with the shield enemies and the block chance Remnants.
Using Splitting Steel as well, if that matters.