Probably nobody noticed that, because this is a minor thing. When opening a world to LAN, there's no colon after the "Game Mode" option. It's this way since a lot of time ago, but, searching for "colon", I still haven't found any similar bug report.

The reason I think this is a valid bug is because the exact thing happened when creating a new world, and it was fixed: MC-36847

Also, since it's a cycle-through button, like any other cycle-through button in the game, there's a colon separating the "field" and the "value"; this is why I reported this bug. Still not sure if it's valid.