I was master working a set of GA to core skills grasp of shadow. At MW rank seven I had 5 ranks to core, at rank 8 I hit the +25% on the core skill and it stayed at 5. I’m no math wizard, but 25% of 5 is 1.25, which would round down to 1. Not the 0 that it gave me.
I’ve had the same issue where the master working bonus on an item stays the same or even gets reduced when you get the 25% increase. Seems only to happen to skills.