screen capture and floating thumbnail

Hi all.

I was enthusiast about the new feature of "floating thumbnail" when you take a screen capture, like it happens with iPhone.

Unfortunately with my new Mojave installation (upgraded from High Sierra, after buying a new MacBook Pro), it never works except few times in which I saw effectively the floating thumbnail. Usually the thumbnail on bottom right does not appear, even if the option is ticked.

I tried obviously to untick and tick again, to remove the preferences for screen capture app, but nothing changes...

Do you have any suggestion or anybody has the same issue?

Thanks

Francesco