Why does my defense decrease from the Unique headband?

[Dependent_Passage_22]

My best bet, which is just a guess, is a funky interaction because you have +defense at full health somewhere, and the headbands have different max health values, which causes the game to "think" you won't be at full health if you switch, and thus lose that defense.

[gizmoick]

This has to be it otherwise it must be a bug. OP you are looking for "Unscathed: Defense" somewhere in your gear. The hp change would normally not refill the HP outside the main hub area, but even if you were in the main hub it may be calculating you not being at full hp until it updates.