When a world is saved after being affected by the common visual glitch caused from outdated AMD drivers, the world's thumbnail also gets messed up. I have solved the AMD glitch by enabling VBOs, however even after that, the world's thumbnail fails to update and still has the screenshot of when the world was saved during the AMD glitch.
MC-99504
Thumbnail glitch caused by AMD driver glitch
Resolved
Invalid
0
Click on the world and choose "Edit", then click "Reset Icon". This will delete the icon saved so you can generate a new one.