The thin line between menu and command bar and its strange behaviour is also visible in the original theme of Windows 10.
It even changes its appearance when changing between a normal folder and a 'picture folder'. As the line is very, very thin in the theme of Windows 10 it's hard to recognize the difference.
In case of a 'normal folder' its properties are defined within
Explorer & Shell > Explorer > Menu Bar
.
In case of a 'picture folder' or 'music folder' the properties can be found in
Toolbars, Headers, & Rebar > Rebar > Rebar
.
But I haven't been able to figure out what actually controls this line.
Well, not so important ;-)