"Multiplayer (3rd party)" shows in window title when disconnecting from a singleplayer world

The bug

1.15.2-pre1 added info about the world/server you're currently playing in to the Minecraft window title. However, when disconnecting from a singleplayer world, the title is always "Mutliplayer (3rd party)"

To reproduce

1. Join a singleplayer world

2. Look at the window title
   → ✔ It says "Minecraft <version> - Singleplayer"

3. Exit the world

4. Look at the window title again
   → ❌ It says "Minecraft <version> - Multiplayer (3rd party)"