Linux

What are you using to full-screen the video?

That is, are you using the letter-F key while the video is the active screen element, or likewise, clicking the ⛶ full-screen icon in the bottom right of the video, or are you doing something else?

What it sounds like you're doing is using something like the F11 key, which is the browser's own "go fullscreen" key, but that's not for the video. (I will admit to having done this accidentally once or twice when I'm not concentrating.)

OR Maybe you're not doing that but something else is hooking into F11 instead??

FWIW, I've never had anything like what you describe happening on Mint (LMDE)

I had this issue once and it was because of a browser extension. Check if it's gone if you disable all. Worth a try.

I recommend using freetube: https://freetubeapp.io/
It's a youtube desktop client, no tracking or ads and works perfectly on Linux.

I have never had this issue before, but you should include which browser you are using and how it was installed. In my experience, "flatpack" browsers sometimes act weird, and installing them the traditional way resolved the issue.