If you crash or log our, the Mage Armor effect on your armor vanishes. This appears to be a recent problem, never noticed it before. PC: William Grafton.
It has been like that for a long time. It is one of those spells that affects an item rather than the user (e.g. magic weapon, flame weapon) so unless you take it off and lay it on the ground, it will be lost when you log out and back in or crash.