Could you please try in a private window / a new profile ?
If that doesn’t happen there, disable extensions or completely remove them.
Also maybe try disabling the vertical tabs flag at brave://flags and then re enabling it.
Same on Debian with Awesome WM. Doesn’t happen always but often enough, not only on YT but on any site that has fullscreen videos. For example, Discord and Element. It can be fixed by pulling the tabs out (I have them auto hide) but yes, it’s annoying. Also, vertical tabs scrolling sometimes glitches, I can’t scroll all the way down or up. If I scroll in the opposite direction a bit, it lets me to scroll to the direction I wanted.
It appears that the problem, though fixed in beta (v54+), is present in the new release (Version 1.56.9 Chromium: 115.0.5790.98 (Official Build) (64-bit)).