I encountered an issue on the pc version of mcc downloaded through the Microsoft store. When trying to switch from controller to mouse and keyboard, clicking the left mouse button would act as if it had clicked on whatever was highlighted, but it wouldn’t actually do anything. The game doesn’t freese, you just can’t do anything with the mouse other than move it around. The mouse also remains on screen during gameplay and there is no option to turn this off. This is the version without Halo 3, I have halo 3 flight on steam atm though.
Make sure and file a bug report about this on the Halo Support site: https://aka.ms/HaloSupport
Be sure to read over the known issues list first before reporting
If you Alt+ Tab out of the game and then focus back on it, does the mouse work then?
What about KB arrow keys?