Assuming you’re using swords, sword mastery should increase DS damage. I can’t think of any situation where points in a mastery would ever decrease damage. At worst they should have no effect if the mastery doesn’t apply to a situation (such as having axe mastery but using sword or something).
Just to try to understand the problem can you confirm:
- You are making no equipment swaps
- You are sitting with the character screen and the skill menu open, and you have unused skill points. As you add skill points, your DS damage is going down.
- You’re doing something like #2 and not doing a full respec - it would be possibly here you’ve made unintended changes.
3.1) If you did a respec, it’s possible something is going on with weapons not being equip-able even when you meet the requirements, if you’re using bonuses from items to meet the requirements. I recall there’s been similar bugs posted where Grief is switching hands on people or otherwise not being considered despite meeting the requirements, but things look normal again once “hard points” are committed.
Just want to make sure something similar to : Barbarian Dual Wield Berserk Bug isn’t happening.
What items are you using and what stats do you have? Might help people figure out what is going on.