When a locator map that has had a custom name applied to it is stored in a container, such as a chest or Shulker Box, after quitting and rejoining a Realm the map will appear in the container as "Unknown Map." Moving the map to the player inventory will restore its name for the remainder of the play session.

This makes retrieving maps from containers rather annoying, as one must either fastidiously arrange them in the container and remember the exact position of the map one wishes to retrieve, or one must move maps to inventory repeatedly to discover the correctly named map by trial and error.

This issue is fully repeatable and is happening for multiple players on our Realm.

We're not sure if the issue was present in 1.16. It's definitely in 1.16.1. It was not present in previous versions.