If you change a game rule, exit the game rule menu, and then re-enter it, the default of that game rule will change to what you set it to.

To recreate this bug:

1. Create a new world.
2. Go into Game Rules.
3. Look at the default of a game rule, then change it.
4. Exit Game Rules.
5. Go back into Game Rules.
6. Look at the default of the game rule you changed, and it will be the value of the change. It will be incorrect, because the correct default is not the change.