You can disable the top bar if you don’t use it, it is just that you need to uncheck the Top Bar menu location if you don’t have a top bar. Look at this demo, there is no top bar: https://simple.oceanwp.org/
Ah, alright, I found my issue. In Menu Locations, the Top Bar selection was empty – just a blank option – instead of -- Select --. So it looked like it wasn’t set, but it was… Maybe I changed a menu name at some point?