I think this blank screen bug is since and only with Firefox 142, as I see in my phone that it was updated 2 days ago.
And I tried Chrome browser with no blank screen bug.
I managed to upload the video, you can see that a small blank bar appears at the top of the screen.
And the bottom address bar goes up to the top, each time the keyboard appears.
Until I force close Firefox.
It seems to be what’s causing the blank screen bug on edit.
More a Firefox bug than a Discourse bug, but I don’t know how to report bugs to Mozilla.
PS. I removed my off-topic MP4 upload bug, I found the reason and work-around.