Umbilicus Immortalis Linen Belt doesn't work with Reavers Rage ability

The ability "Minions cannot die while affected by a life flask" doesn't work when you use the Rage ability. I know they are under the effect of a life flask but they still die. Only happens when you use the ability
Is this a bug or intentional?

So far it seems to work only on enemy attacks. Doesn't work with Infernal Legion either, they still just die.

One thing I've noticed with the Rage ability is that it also disables the healing from Immortalis after some delay, I could chug multiple flasks (incl. instant recovery) at once and their health would only go down.