This is a duplicate of MC-241849, however I have new information that can help reproduce this bug. And I have experienced on more than one device(windows 11 gaming PC and windows 10 slow laptop). The icons seem to not reset properly if you open and exit the world after resetting the icon, unless you have relaunched minecraft since the last time you reset the icon without joining that world. Also relaunching minecraft seems to fix the icons displayed if they were not correctly displayed when you last exited minecraft. 

I also experienced at least other 1 bug while recreating this bug, but that bug has already been reported. 

I recorded a short video and a long video, which will be linked here: https://youtu.be/H0TSsU3VQqk 

steps to reproduce: 

Make a new world, then quickly leave the world, but let the world automatically save while you're in the pause menu first. (if you don't let it autosave the world icon will not save)

Reset that world's icon in it's settings.

close minecraft and then relaunch it.

open the world that has no icon and then leave it. The icon will correctly change.

reset the icon again without closing minecraft. In my situation, the world listed below it that will later share this world's icon, now shares the icon of the world below itself. 

open the world and then quickly leave again, after letting it auto save in the pause menu. The world Icon will now match a the icon of the world below it.