It’s probably because your device can load chunks fast enough.
Still an issue 1.19.73
The iOS photos app will give .jpeg files not .png files.
Also on 1.19.73
It works if you put the pack into the minecraft folder.
Probably caused by the app not requesting permission to access the location of the file.
Using a shortcut or notes works because it gives minecraft access to the file.
Steps to replicate
Open minecraft
Open video settings
select Change screen safe area
I’m new to the bug report system and a did the wrong version.
That’s intended.