Consider the following test file:
https://jyavenard.github.io/htmltests/tests/mse_mp4/bipbop_fullscreen.html

The page has a video element within a <div></div>

It has two buttons, one will call requestFullscreen() on the video element, the other on the div parent element instead.

STR: 
On an iPad:
1- Load the page https://jyavenard.github.io/htmltests/tests/mse_mp4/bipbop_fullscreen.html
2- click on the button "video fullscreen"
3- now press the PiP button in the top left corner
4- tap on the "video fullscreen button again"

Now, notice that after step 2, no banner "jyavenard.github.io is in fullscreen is shown"
and step 4 does nothing.

Now we repeat the above steps, but this time with the "Div fullscreen" button
5- Load the page https://jyavenard.github.io/htmltests/tests/mse_mp4/bipbop_fullscreen.html
6- click on the button "video fullscreen"
7- now press the PiP button in the top left corner
8- tap on the "video fullscreen button again"

Now, notice that after step 6, the banner "jyavenard.github.io is in fullscreen" is shown
and step 8 the element goes to full screen.

From a user perspective, the end result is the same: the video plays in full screen. However, depending on which element the JS called to go into fullscreen, there are subtle differences that I believe shouldn't exist.