So you click on Help in this situation, and nothing happens? In fact, simply moving the cursor to Help should cause the menu to follow. I suppose that if there's a problem “on our side”, it's more likely to be with the toolkit, i.e. GTK.
A few questions:
Is it reproducible with a compositor other than KDE's (I can't reproduce this with labwc or wayfire)?
Is it reproducible with another Xfce application, such as thunar or ristretto or xfce4-terminal?
Is it reproducible with another non-Xfce GTK application, such as gedit or gnome-terminal (no menu bar in this case, though...)?
Is it reproducible with Qt apps (vlc, qbittorrent...)?
It is reproducible with Xournal++, sublime text, MyPaint, Thunar so I'm not sure which toolkit is the one having issues, unless there is multiple.
I have not seen this problem mentioned on any other DE. As for a compositor I have not tested anything but the KDE default.
There has to be menu bar to experience the bug, so it can't happen in gedit and other applications without a traditional menu bar.