Same problem here, the custom PNG doesn't work anymore. It appears when I move the mouse quickly but disappears.
This is the link to a page of my website for an example:
https://www.clara-andrea.com/tipik/
Hey Patrick, I tried to replicate this but for me it worked. Maybe there are some special circumstances with your site and that's why it didn't work. If you like you can send me your wordpress admin panel login data with a link to this post to armin.unruh@gmail.com Then I could temporarily activate cover for phone and test it myself.