The bug itself is simple, but strange.

Even with neutral affinity and fully booked 100% chance, Mighty Ukko's -ACC and Block Buffs effects from his A2 don't get applied to Scarab, whether it has buffs that get stripped, or no buffs to begin with. No resists, the effects just 100% don't work against the boss.

My conclusion is that the boss' damage reduction mechanic is actually implemented as a hidden, unremovable buff which makes Ukko's ability bug out.