If the bleed has a damage associated, he will never take 0 damage from it.Also, there is a node on that fight which enhances bleed damage, meaning that your 90% (+ coagulate) bleed damage reduction will be drastically lower
Omega red reduces bleed damage by 90%. By default, the damage is set to 100% of the stated value. If it gets boosted to 150% damage for example, Omega Red will still take 60% bleed damage of the original value
An answere from a mod would be nice on that.