After purchasing a woodland or ocean explorer map from a villager, going into the nether or end will change the map to a blank map with a "unknown map" description. This map will stay blank until logging out of the world and logging back in again.

If a villager with an explorer map trade is taken from the overworld to the end, buying maps from him there and going through the exit portal will not turn the maps blank.

To reproduce

1. Buy a explorer map that has an ocean monument or woodland mansion icon on it from a cartographer
2. Enter the Nether