Fullscreen video in Safari on Catalina

Are you running any anti - virus programs? Using a VPN?

Try setting up another admin user account in System Preferences/Users & Groups to see if the same problem continues. Please post back on whether or not this worked. Also try the Safe Mode. Please post back on whether or not this worked.

Isolating an issue by using another user account 

Safe Mode - About

If it works in the Safe Mode, try running this program when booted normally and then copy and paste the output in a reply. The program was created by Etresoft, a frequent contributor.  Please use copy and paste as screen shots can be hard to read. Click “Share Report” button in the toolbar, select “Copy to Clipboard” and then paste into a reply. This will show what is running on your computer. No personal information is shown. If the log won’t post, try posting it in Pastebin and provide a link in a reply. After pasting the report in a PasteBin page, go to the top of the page, and copy the address in the URL bar. Paste that in a new reply.        Pastebin

Etrecheck – System Information

Web site version if required.  https://etrecheck.com

Most posters recommend not using cleaning programs. They can destroy normal computer operations. I suggest you uninstall CleanMyMac 4 using the Developer's uninstall instructions.

Try going to System Preferences/Users & Groups and remove all your login items. Restart the computer, do not open any other programs except for Safari, and test.

I have exactly the same issue.

It is completely random. Sometimes it does it, then on the next video it won't. Or it will do it for 20 consecutive YouTube videos, then not do it. It is very strange.....

I've been having the same problem with my full screen mode on Mac OS Catalina when going in full screen mode and the updates still hasn't fixed this issue.