I can totally confirm this. When I was more serious about music, I’d practice something but totally misinterpret it, so the effect would be worse than if I had not practiced at all. The same thing was true for school work. I’d hyperfocus on some little detail and lose sight of the big picture, but when I just phoned it in I did well.
With music, I find it hard to be goal-driven. The goals aren’t easily defined. The music either sounds good or it doesn’t, and there are so many components to “sounds good” that sounding good isn’t even a useful metric. I suppose I could practice scales and articulation and little etudes, or go over the more challenging parts of a piece, but then the piece needs to be run through in full so I know I can play the challenging parts in context, with transition.
Musical practice is more free-form for me. I won’t know the challenging parts of a piece until I get there, and when I do I will have to practice those over and over again, just because that’s how my personality is. Then I will run though the piece just to see if everything I’ve practiced holds up in context. If it does, then great. If not, then it’s because I’ve overrehearsed the hard parts, and we’re back to the drawing board 
