This bug has been introduced ever since one of the 1.16 pre releases. The first icon would temporarily look identical.
The steps to reliably reproduce this are very complicated. I'll try to do further testing to explain how to reproduce this consistently, however following what I did in my attachment should consistently reproduce this bug as well hopefully.
Despite the title, I've gotten in situation where pressing Save in Edit world actually fixes it, whereas pressing cancel or exiting singleplayer and coming back breaks it.
If you only have three worlds in this order: The world on the top of the list gets a new icon, the world under that is pack.png and the bottom world has an icon previously, after you get the new icon the bottom world will have the same icon. If you then delete the top icon the bottom world will have a blank white icon.