It seems that Molt Augmented is the issue. I've had a similar issue when using Augmented and Power Drain together.
If I take Molt Augmented off and activate Power Drain, it applies to Vex Armor. If I have both Augmented and Power Drain on, the ability won't update at all until I either let it expire or jump into a hole.
Normally, this isn't much of an issue, but with the Guardian Armor augment now being a thing, someone who doesn't know about this bug will have no idea why their buffs aren't increasing in power.