Unable to generate Fortify stacks during Ultimatum encounters.

I don't have anything to help explain this, but I noticed that I don't generate Fortify stacks while in Ultimatum. I am using Lacerate of Haemorrhage, have Fortify support equipped, and I never generate Fortify stacks against Ultimatum monsters/during the encounter, despite dealing sufficient damage.

I am unsure if it makes a noticeable difference, or if it does generate stacks and it's just invisible on my buff bar, or if I really don't do enough damage, but I have frequently glanced at the bar to see no Fortify stacks, and there is no effect around my character. I will acquire clips to demonstrate later, if needed.

Here is a demonstration:

https://youtu.be/MsOkdASAa48

Despite having Resolute Technique, meaning I always hit, my Fortify stacks start to dwindle in Round 8, then become extremely scarce in Round 9, and then no longer generate in Round 10. I am damaging both rare monsters, which are harder to generate Fortify off of, and common monsters, which should generate at least 10 Fortify stacks on hit.