I have a +1 armour and +1 bracers, both of which give the "armour ac bonus".
When I wear the armour and the bracers together my AC is 1 lower than if I wear the armour without the bracers.
Armor on bracers is the bug. You should report the item, and where you got it.
I don't think this happens in vanilla nwn, which takes the highest AC Armor Modifier without stacking. It feels like there's a script in place to counteract a nonexistent problem of AC Armor Modifier stacking, and this results in the lower AC Armor Modifier value being subtracted from the higher one (I have since confirmed that +4 from Mage Armor also becomes a +3 in the face of the bracers).
While manually removing +1 from every bracer would fix the problem in the short term, identifying the offending script and removing it would allow +1 bracers to be made in future, without each one being hunted down and removed as they are made.
In the meantime, though, I have reported the bracers in question.
In standard nwn, a bracer of +1 armour stacks with a mundane full plate.
There is a script in efu that prevents that from happening. +1 armour bracers are only meant to give ac if you are wearing cloth in efu.
The script is probably not prepared to deal with your original situation, and causes issues.