Okay, I reproduced it again. I created a terrain and applied some splats on it. Everything was fine. Then I applied CMU shader and I started painting and that's when those areas went shiny. Also should note that only splats from 4-7 layers tend to have this problem. Even those that were absolutely rough on standart shader turn shiny when painting on them with CMU on. Changing specular values doesn't help. That terrain in the demo scene also gets shiny. It's a bug alright. Maybe it's got something to do with the new texture types in 5.5? Oh, I'm on 5.5 by the way.