A re-created Hardcore game isn't Hardcore unless you cycle through game modes and reset it to Hardcore

The bug

A re-created Hardcore game isn't Hardcore (despite the 'Game Mode' button displaying 'Hardcore') unless you cycle through game modes and reset it to Hardcore.

How to reproduce

1. Select a hardcore game and then 'Re-Create'
2. Don't change the game mode settings
3. Select 'Create New World' button
   → Notice that a survival world has been created even though the 'Game Mode' button said 'Hardcore'

Code analysis

Code analysis by Marcono1234 can be found in this comment.