Here is an update. I figured out the problem, but I don’t know what to do about it.
It seems that if a project contains a test that has either randomized questions or is a timed test, the full screen option on any YouTube videos in the project is disabled. This must be a glitch as I can’t imagine there being any deliberate reason for it.
Here is another video that shows the problem https://youtu.be/HKotM6xs1jI
If anyone has any suggestions, they would be appreciated.