Possible bug on Training Grounds (Hit Master x Grudge)

I understand that Grudge by all math should be a dmg increase on Hit Master. Yesterday I went on training grounds to see what actually would look like on my build if exchange Hit Master for Grudge and the results were… a 10-15% dmg loss on my heavy hitter. Like from 2.6M to 2.2-2.3
I was just using the engraving service and using my own stats. I think it might be a bug since i can’t think of another reason to get this dmg loss. Yes I’m sure that other settings were the same, just replaced engravings.
Then comes another question, is it only on training grounds or is grudge bugged altogether.

Anyone else experienced something similar?