When creating an app with the App Studio and checking "Fullscreen Mode" under 1 Details / App details / Advanced I get the follow error message under 3 Finish / Test and distribute:
"isFullScreen | Property "isFullScreen" has not been defined and the schema does not allow additional properties."
In the app manifest preview I can see a line stating ""isFullScreen": true", but it has no effect when I install the app.
This has been reproduced on three separate tenants by three different users on three different machines.