This is about Jetpack's "Shortcode Embeds" module.
When viewing an embedded Youtube on a post, the Full Screen button usually on the bottom corner of the video is missing. This forces the reader to click on the Youtube button and away from the blog post to view the video in fullscreen.
The issue only appears if the reader is using Chrome and is logged into their Youtube account. Tested on Mozilla and IE, no problem.
I did not make any changes, I think Youtube recently edited their embed code and did something with the fullscreen variable, can the team at Jetpack update the code please?