I can confirm this behaviour on all 3 of my boards (2 Gloomy, 1 Duet). Tested with all internal and external plugins disabled just to be sure.
It's repeatable by hitting refresh on the tab and immediately switching tabs and waiting a couple of seconds, then tabbing back to a grey screen. I should also note that if the page is even partially rendered before tabbing off, the issue does not manifest.
I can not replicate this on firefox - only chromium based browsers (running on windows).
Notionally it seems to be influenced by the blue "connecting" dialog - if you go back to the duet tab before it closes the "connecting" dialog, the problem does not seem to manifest - but this could just be a red herring.