Mage Armor does not work with +1 tunics. Instead it gives a total bonus of only +3. Mage Armor cast on +2 tunics gives no bonus at all. I suspect that Mage Armor cast on +3 tunics would actually decrease AC. I suspect that Mage armor is scripted to subtract any magical AC bonus, but as they don't actually stack the net AC is decreased.

The higher of the two magical effects is the one affecting the creature's AC, I think, so your AC will not actually go down.

For example, a Padded Armor +1 would look like this: Base AC: 1 Magical AC: 1 Magical AC (Temporary): 2

Only the temporary effect, because it's higher, is taken into consideration (while it exists), for a net AC bonus of 3.

I don't know if it's desirable to change this behavior or to leave it as it is. I do know that I'm sick of having to look at this little 1st-level spell.

Brea's right! This should now only take into account the armour's base AC, since temporary effects don't stack anyway.

Padded armor +1 should now be: Base AC: 1 Magical AC: 1 Mage Armor (temporary): +3 Total AC: 4 (+1 base, +3 temporary)

You also should no longer be able to suffer an AC penalty.