That can happen, but I find that if you separate the sheets as they are going in, and kind of scrunch up the fitted sheets and duvet covers so they don’t start wrapped around the flat sheets or blankets, then they are far less likely to ball up in the wash.

I had a roommate in college that would wrap his sheets in the fitted sheet (along with other laundry) and throw it all in like a package. His laundry always smelled like mildew.