I can reproduce this. A workaround is to skip the "zoom in" step, or zoom back out before entering fullscreen mode. Marking as S3 since a workaround exists, but this looks pretty bad on a popular website so I'm also adding it to the APZ diagnosis backlog. Note also that once in fullscreen mode, YouTube is also affected by bug 1889017, which seems like a more severe problem.
Bug 1875595 Comment 4 Edit History
Note: The actual edited comment in the bug view page will always show the original commenter’s name and original timestamp.
I can reproduce this (tested on Samsung Galaxy Tab A9+). A workaround is to skip the "zoom in" step, or zoom back out before entering fullscreen mode. Marking as S3 since a workaround exists, but this looks pretty bad on a popular website so I'm also adding it to the APZ diagnosis backlog. Note also that once in fullscreen mode, YouTube is also affected by bug 1889017, which seems like a more severe problem.