Critical avoidance calculations

Can anyone explain how the game calculates critical avoidance v critical chance/critical chance up? I assumed that it was a straight up resuction in critical chance, but I’ve seen yoda crit on HK several times now (even though he has 150% critical avoidance). Is critical avoidance incapable of reducing critical chance gained from buffs, or is there a base crit chance that you can’t be reduced below, or is there a bug in HK’s unique?