Unless it's a hidden modifier, my deflection doesn't change when wielding a two handed weapon or a one handed weapon.

Change where?

 

Check the combat log - that is the most accurate way of checking whether an accuracy or defense bonus is applied.

On the stats sheet on your inventory screen, where it lists all of the defenses. Deflection stays the same.

Yeah, but is it working in the combat log ?

Obviously it not being added to the inventory sheet is a bug, but the value likely is applied in the combat log for attack rolls.

Ah, you're right. It does reflect it in the combat log. So, the problem is simply the display.