You are right, @weaver will have to look it up.
I believe you can fix the bug right now by adding the rule below in Main Options > Fonts &custom > Custom CSS Rule box
.is-menu-mobile .wvrx-menu > li {float:none;}
@weaver, there is a rule
.is-menu-default .wvrx-menu > li {float:left;}
That ends up being applied to the slide open mobile menu and causing the issue. It is applied because on mobile, the body tag has both classes, .is-menu-default and .is-menu-mobile