Someone has disappeared from the map

Gone entirely is different from frozen.

Vanishing completely and sitting still are different problems with different causes. Worth telling them apart before worrying.

They left the space

Leaving removes someone from the map immediately, with no announcement. If somebody is gone rather than stationary, this is the most likely reason.

They were removed

The space's owner can remove a member. The effect looks the same from the outside.

They deleted their account

An account pending deletion is signed out and stops appearing. There is a 30 day window in which signing back in restores everything, so someone can reappear days later. See Deleting your account.

They are still there but frozen

If you can still see them and the position is old, that is a different question. See Someone's location is not updating.