Question:

When viewing split screen, full screen mode does not work on YouTube app.

Answer:

We confirmed that there may occur full screen mode not working on the YouTube app while using split screen view because of some compatibility issue.

Relevant issue is reported to Google team, and there will be update version of YouTube app soon.
Please update with the latest YouTube app when it is available.

As a temporary workaround, you can run and try 'Open in split screen view' with the YouTube app again in main screen.
* Path for temporary workaround : Main screen > run YouTube app > tab 'Recents' button > tab app icon > tab 'Open in split screen view' > run other apps(internet app, etc.)