I've got this loxodon, monk/fighter that I put the very rare version of the barrier tat on.  It isn't wearring any armor, so I don't get why the AC is using the monk's unarmored def instead of the tat's when the barrier tattoo is 4 AC higher.

Is the tattoo attuned? (ui is either in the sidebar when you click on the tattoo, or at the bottom of the inventory screen)

Also, remember that the very rare barrier tattoo doesn't add dexterity to AC.

The description of the tattoo says that it makes your AC 18.  Mine is currently 14.  It is the only item I have attuned.

Sounds like a bug.

I added the Barrier Tattoos to a test character and it appeared to be working.  Can you try removing the item and re-adding it to see if that helps.

Yup, that fixed it.  Thanks!