the icons seem to not reset properly if you exit the world shortly after entering it after resetting the icon, unless you have relaunched minecraft since the last time you reset the icon without joining that world. 

steps to reproduce: 

reset icon for a world in the world'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 that will later share this world's icon, now shares the icon of the world below it. 

open the world and then leave again. The world Icon will now match a different world's icon. In my situation, its the icon of the world below it which no longer shares the icon of the world below itself.