The bug
Players holding a copy of a particular map will still appear on the map even when logged out.
How to reproduce
Craft and clone a map
Hand one copy over to another player. You should see two white cursors on the map now, one for you and one for them.
Have the other player log out. Their white cursor will remain where it was.
Related issues
relates to
Attachments
Comments

I can confirm this.
Is this still a concern in the current Minecraft version? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.
No answer on querry, assuming no longer an issue.

This should be fixed now.
Nope, still an issue for 1.13 and 18w30b.
Confirmed for 18w31a.

MC-134273 was marked as fixed and it's supposed to be this same issue, so I assumed it was fixed now, as the only place I have to test is my server and I can't update for the snapshots.

@unknown, we were not able to reproduce following the provided reproduction steps. Did you reproduce it on a dedicated server or maybe did you play in LAN mode? Or did you do something else to reproduce?
Fixed for 18w32a.
@unknown: I was able to reproduce this in a LAN world in Gamemode & Cheats (On). Now I cannot reproduce it anymore.
Fixed for 18w33a.