Check the Screens. The TG T20 Myth Offhand gives -10% Damage Bonus.
When removing every single thing that gives DMG Bonus it says 10% in the info window. It should show as -10%. When equipping a single anc gear it goes down to 5% or up to 5% hard to say. My guess is it calculates correctly but its not showing negative as negative. But i would like too see a clarification. Maybe having zero % and using the shield thats supposed to reduce it actually increases your dmg when you are at zero % without it. Also regarding the Turn Rate calculation it could be a big increase for speed based builds using mostly a set and poly with that offhand if you are able to get negative Bonus in your dmg increase cause it lowers the power value and and that means a higher turn rate.
Hopefully we can get a trustful clarification if negative bonuses are possible and also working applied ingame.
in game with that debuff
it's simply also a thing of them not adding all of this stuff your character info panel