Steps to reproduce:
1. Open attached project in Unity and in explorer
2. Find and run "Run.exe" (build) in explorer and press 'Play'
3. Run game in Editor and press 'Play'
-- Notice that both are connected to the same match. Standalone build is Host and Editor is Client
4. Disconnect Host by pressing 'X'
-- The editor (previous client) is now a host
5. Press 'Play' in build

Results: Standalone build does not find existing game instance and creates it's own.

Note: Does not reproduces if LAN matchmaking is used via NetworkManager UI.

Reproduced with: 5.4.0b22