I had this same problem. Was it an old Radeon card? I don't remember the model, but Microsoft's website said it was too old, so it only has basic support from W

I had this same problem. Was it an old Radeon card? I don't remember the model, but Microsoft's website said it was too old, so it only has basic support from Windows 10. That means black bars. Drove an ultrawide monitor in Linux just fine.