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)"