Enemies spend multiple evasions

I had one of my characters who has Grit with the 2 damage immunities, and another character who has the skill to give evasion provide 3 evasion.

That one character should now have 2 immunities and 3 evasion, yet sometimes when an enemy attacks, I lost 2 evasions and 1 immunity on a single attack.

It seems to be from the enemies who attack with two volleys (which should count only as a single attack). As I understand it, the damage immunity should only be spent if I actually take damage, so a character having 3 evasion and 2 immunity should spend the 3 evasions first on 3 separate attacks, then 2 immunities on 2 attacks; essentially negating 5 enemies.

In practice, sometimes one attack will cancel out 2-4 of my evasion/immunity and leave me exposed. That seems like either the attacks with multiple volleys don't get counted as a single attack, or there is some other race-condition or jitter in the hit detection (schmitt trigger issue?)