I think it is a bug. I had it happen to my account which I've gotten grandfathered from Google Music -> Red -> Premium. The app suddenly changed from saying P

I think it is a bug. I had it happen to my account which I've gotten grandfathered from Google Music -> Red -> Premium. The app suddenly changed from saying Premium to saying Youtube in the upper left corner and I was completely blasted with ads for Premium. I contacted them through Google One and whoever they routed it to got it fixed.