When quitting the game with CMD-Q, the game crashes. This also prevents the world from saving which means that very recent progress is not saved.

STR:

1. Launch Minecraft
2. Start a new singleplayer world OR join a multiplayer game
3. Press CMD-Q when the game has started
4. Note game crash.

This happens consistently in my testing.

This happens when a game is loaded in either singleplayer or multiplayer.

If a game is loaded in singleplayer, this will prevent the world from saving. You can test this by breaking a block just before pressing CMD-Q and then reloading the world. You'll see that the block you broke has returned.

The game does not produce a crash report in ~/Library/Application\ Support/minecraft/crash-reports.

If I enable the game console via the launcher, the following lines are output just before the crash:

This seems to indicate that the game is making its way through the shutdown sequence but crashing at some point.

I've generated a manual crash report in case it is of any use but please note this is NOT the crash report from the actual game crash observed.