For example if i'm wearing my mage gear including magic clothing and I cast the spell, I get the armour increase ok. I then change quip to my platemail and increase is still there.
But then when I change back to mage gear to cast a spell or what not the ar increase vanishes and doesn't come back

So for the moment I just leave my apron of protection at home and my ar increases stays for the whole duration of the spell regardless of how many quip changes.