Mage Armor: +4 Armor AC.

When I cast Mage Armor on a standard tunic (no enhancement), everything works properly and I get +4 AC

When I cast Mage Armor while wearing the seeker arcanist's tunic (with a +1 AC enhancement), Mage Armor has no effect.

My understanding was that the spell is intended to give a bonus of +4 Armor Class, not Armor Enhancement, meaning it should stack with a +1 enhancement on a tunic.

That's not quite correct.

Wearing the +1 seeker tunic with a +1 amulet/+1 cloak, I cast Mage Armor and get a +2 bonus.

Removing the tunic and putting it back on makes the Mage Armor spell entirely disappear, with a message about the same armor type not being able to stack.

Wearing a non magical tunic with +1 amulet/cloak (natural/deflection), I get a +4 bonus.

I believe it should work as if you're wearing AC4 armor, regardless of anything else. Arkov? Howland? Bueller(illum)?

It's finicky. Best advice right now is to not switch armor/clothing after casting it.

I'll put together a more complete response once I get a chance to look over the actual scripts in the mod again.

The issue with casting mage armor while wearing enchanted robes should be fixed in v820.