Clients fail to load in Standalone mode with 2 players while Use Single Process is disabled

Clients fail to load in Standalone mode with 2 players while Use Single Process is disabled.

Note: double checked this, and I cannot repro in Multiplayer Shootout in 4.9. The same issue still occurs in the Marketplace content project Generic Shooter.

Reproduced in 4.8.1 and Main (//depot/UE4/Promotable-CL-2605171). This is a regression: the issue did not occur in 4.7.6

1. Open the Multiplayer Shootout project
2. Disable Use Single Process in Editor Preferences > Play
3. Switch Editor Multiplayer Mode to Play As Listen Server
4. PIE in Standalone with 2 players

Result:
The Client does not load and gives a warning pop-up: "The map specified on the commandline '/Game/Maps/MainMenu' could not be found. Exiting."