If a world is generated using The Void Superflat preset, or similarly empty custom presets, no World Icon is generated. This also occurs if the world icon is reset on an older world using these presets.

This also occurs even in regular worlds if the World Icon is reset while the player is staring into the Void in the End, or staring into the sky in the overworld with Clouds turned off.

Steps to Reproduce:

Create a custom Superflat world in Creative Mode using The Void preset. Once the world is loaded, Save & Quit, and observe that there is no world icon.