The issue you are running into has to do with order of operations.

Within NetworkManager.HandleRawTransportPoll where it is handling the transport disconnect event:

case NetworkEvent.Disconnect:
#if DEVELOPMENT_BUILD || UNITY_EDITOR
s_TransportDisconnect.Begin();
#endif
clientId = TransportIdCleanUp(clientId, transportId);

if (NetworkLog.CurrentLogLevel <= LogLevel.Developer)
{
NetworkLog.LogInfo($"Disconnect Event From {clientId}");
}

OnClientDisconnectCallback?.Invoke(clientId); <--- the client disconnected callback is invoked

if (IsServer)
{
OnClientDisconnectFromServer(clientId); <----- this is where the disconnected client's keypair entry is removed
}
This specific order of operations was from a previous bug where a user still wanted to access the disconnected client's NetworkClient from the ConnectedClients dictionary.

The issue with the code:

protected virtual void OnClientDisconnect(ulong clientId)
{
if (IsServer)
{
Debug.Log($"Server: Disconnected {clientId}");
UpdateTextConnectedAmountClientRpc(NetworkManager.Singleton.ConnectedClients.Count);
UpdateTextMaxAmountClientRpc(NetworkManager.Singleton.ConnectedClients.Count);
}
}
It is using the full count of the ConnectedClients dictionary
So, the two ways to do this and always get the right count:
* Subtract 1 from the total connected clients list

- Or optionally filter out the client id for the client that was disconnected from that list (i.e. use Linq Where or the like)
- Set a bool on the server side that during the update (server-side) if the bool is set it sends the updated count via RPC
- By the time the update is invoked, the connected client dictionary count will reflect the correct value.

We will better document that on the server side the disconnected client's ConnectedClients keypair entry will still exist upon the OnClientDisconnected being invoked.