There may be no solution, so this is more a warning than anything. Before a quest my barbarian had Magic Vestment cast on them while wearing a 0 AC armor. The spell lowered my AC by two points (I just happened to be looking at my character sheet and I saw the AC drop immediately as the spell was cast). It was not solvable by resting, relogging, resetting the perk, reloading a different area or a DM killing and rezzing my PC.
Oddly, it just seemed to eventually sort itself out and all looks fine now, but it was all triggered by the Magic Vestment spell.
GSID: SavageFey
Character Name: Imogen Wolf
As with all item property "buffs", the most recent will replace the previous effect. Your perk provides an item property bonus of +4 armor AC which is replaced with the item property bonus of +2 from the spell.
This same effect happens when you bless weapon of magic weapon, flame weapon over a different flame weapon, greater magic weapon over magic weapon, etc.
As you said, a good warning and probably nothing that can be fixed.
I had thought that to be the case as well when I saw the AC drop. However, when I dispelled beneficial effect, rested, was DM killed...none of these solved the problem, the AC was still far too low which is why I posted it as a bug. But yeah, I don't expect it to be something that can be fixed, only something to watch out for if you've got this perk.
Item properties do not fade with dispel magic or death. They fade on the creator of the effect resting, expiring, or logging out. :P
Next time I'll kill RwG then, good to know.
Move to another area. This is what fixed it.