This is nothing new, it is a drawing glitch that is visible since the first day ShellDimScreen() was introduced to ReactOS in SVN r72573.
The glitch does only happen when classic theme is selected, but it does not matter whether the ThemeSvc in running then or not.
Symptom: part of the Taskbar frame is not properly repainted when ShellDimScreen() does its screenshot and therefore painting remainings of the (already collapsed) startmenu stay visible
The only thing that gets logged, is most likely unrelated:
The bug is never visible when a "real theme" like Lautus is selected.
I suspect a bug in WIN32SS.