When you open a UI like advancements, Esc menu, command blocks or signs the game gets paused in singleplayer, however this does not happen when opening the game mode switcher.
How to reproduce:
1. Play singleplayer.
2. Go up some blocks.
3. Start falling.
4. Open game mode switcher.
What happened: You still fall which means that the game wasn’t paused while game mode switcher was open.
What I expected: The game gets paused while game mode switcher is open.