Label set with $WS_EX_TOPMOST and $GUI_ONTOP is hidden when I mouse over control.

I have a button over top of which I want to sometimes place a label.  I set the label's extended style to $WS_EX_TOPMOST.  However, the problem is when I hover over that button, the label is then hidden behind the button.  This seems counter intuitive.

Is this an issue with Autoit or something I'm doing wrong?  Is there a workaround?  Finally, should this be reported as a bug?

#include <GuiConstantsEx.au3>
#include <StaticConstants.au3>
#include <WindowsConstants.au3>
 
Const $SIZENWSE = 12
 
GUICreate("Repro")
 
GUICtrlCreateButton("Hover Over Me!", 10, 10, 90, 90)
 
Const $h_box = GuiCtrlCreateLabel('', 8, 8, 15, 15, $SS_BLACKRECT, $WS_EX_TOPMOST)
 
GUICtrlSetState($h_box, $GUI_ONTOP)
 
GuiCtrlSetCursor($h_box, $SIZENWSE)
 
GuiSetState(@SW_SHOWNORMAL)
 
Do
    Switch GUIGetMsg()            
        Case $GUI_EVENT_CLOSE
            Exit
    EndSwitch
Until False

Edit: latest beta on Windows 7 x64.