Hi,
I have been having this issue since last week.
Initially when loading an app in the Studio the first screen renders correctly. When I switch between screens, everything goes grey and no objects are visible (on any screen). Even if I switch back to the initial default screen, this also shows just a grey screen.
After first opening the app:

After switching between screens:

I believe this is connected with the new version of the PowerApps Studio last week.
Is anyone else having this issue?