1- Open an app and put it in fullscreen.
2- Go to a new workspace (Super+Right)
3- Go to workspace overview (Super+Down), press left and select the fullscreen workspace.
Result:
The wingpanel is shown for a brief moment after point "3"
The wingpanel is not shown on the top of the screen If Super+Down is pressed on the fullscreen workspace and no other workspace is selected before selecting the fullscreen workspace again.