This question is just something that I want to know (nothing is broken in code yay)
In the most browsers going to fullscreen causes the security adnotation coming (in most cases) from the top of the display.
I can not understand why it is suspicous, could you share your knowledge about why it is less secure to view (fe) the video in fullscreen mode?
The browser chrome is a trusted security indicator, and full-screen mode hides that indicator.
If a page could enter full-screen mode without the browser notifying the user, a malicious page could enter full-screen mode, draw Chrome's address bar, and look exactly like a Google login page to trick the user into giving their password.