I think that fortification is bugged in Ultimatums

The only place I cannot seem to get stacks of fortification properly is while doing ultimatums.
I can easily get my 20 stacks even versus boss enemies but for some reason I can't get max stacks even when constantly hitting tank ultimatum rares instead I have trouble maintaining 10-12 at best.
I am using lacerate of haemorrage on a gladiator
I have resolute technique so no, they are not dodging my attacks.


The effect of not gaining forification properly seems to get worse the higher you get in ultimatums.
In the earlier stages it seems normal but after half way it starts to have issue,
I'm very tanky and just stand there spamming lacerate but can't get stacks properly at the later stages