The issues on disaplay are:

Duplication on StopHost

The Unity Networking system is creating a new instance of the LobbyManager (inherits NetworkLobbyManager.cs)

When LobbyManager.StopHost is called by OnDisconnect from the LobbySlotsPanel, a new instance of the LobbyManager appears in the scene.

Duplication issue

Am I supposed to Destroy the lobby object here, and why?

UI elements misplaced on Canvas

This issue appears when a new LobbyPlayer is spawned and then parented the slot. Notice how it works fine on the host, but is misplaced on the clients.

Canvas issue

(A simple fix to this is to set the transform.localPosition of the LobbyPlayer to on start, but that is a hack and it should be working in the first place.)