Thanks Weaver, that helped. I had “Rounded Primary Sub-Menus” ticked in my Menus appearance panel. I’ve unticked that, and my problem is solved. I didn’t like the rounded corners anyway, and have had it in mind for several weeks to get rid of them! So, all good for me. The bug, of course, still exists.
I really appreciate everyone’s help. A merry Christmas to you all!
Karl