Baldur's Gate : The Sword Coast Chronicles : NWN2 Persistent World

Might be related to that (from latest server updates):

Revision: 1856
Author: Valefort
Date: Sunday, March 25, 2018 5:11:32 PM
Message: Some mostly cosmetic cleanup on tempest whirlwind.----

My Bard/Pirate/Tempest/WM does not always get his +2ac from tempest defense (when dual wielding).

Sometimes it works, but often it doesn't. Never noticed this issue before.

Seems related to weapon swapping and armor swapping.
Is tempest defense armor bonus now ?

No it's dodge AC (or whatever it was, it wasn't touched), and anyway this change is on tempest whirlwind, the feat.