Update….
I finally found the cause of the problem (at least in my case).
The culprit was : “Steinberg Audio Power” scheme !!! 
In Studio setup I had “Steinberg Audio Power” scheme activated ,as suggested.
Very strangely this was the problem…
I deactivated this option and activated in Windows Setup>Power & Sleep>Additional power settings>High Performance instead and miracle… the droupout problem on first playback disappeared.
Now Cubase works as it supposed to be without any audio dropout issue.
For me this fixes this issue. Hope this could helps others. 