VST instrument's window frame appear and disappear

I am facing the following problem with every VST instruments
In unsolicited time, plugin’s window frame disappears/closes.
When I click with the mouse anywhere in Cubase’s window frame it reappears again.
From a quick search I did, I found out that something goes wrong with the setting of “Always on top”.
When I unchecked it the problem of the window’s disappearance it stops, and plugin’s window frame remains open.
Ιf I check it the problem comes back.

Any suggestion?
Please let me know