Using the default YouTube embed code, you aren't able to successfully embed a YouTube video in full screen format.
The way the embed code is set up, it isn't passing HTML validation with the editor.
allowfullscreen value in the string with ="true".
Example of non-working code from YouTube (under "Share" > "Embed"):
Example with change that resolves the problem:
This change lets the embed code pass HTML validation within a content block.