PaperDollFrame seems to just register all the stat events including UNIT_AURA and flag an update through OnUpdate.
https://www.townlong-yak.com/framexm...lFrame.lua#383
https://www.townlong-yak.com/framexm...lFrame.lua#397
I'm only seeing UNIT_RESISTANCE fire from toggling Mage Armor on Classic. I don't know of any similar spells on Retail that I can test this with to see if it's just disabled on Classic or if Blizzard is piggybacking off other stats updating.
Note: Mage armor does have a resistance component, so watching only that isn't ideal either.