I had no issues with the proximity sensor in MIUI11, and started tot use 12 on Friday.
so far I have some new issues:
- proximity sensor during calls does not work correct
- lock screen in Dark  mode is looking awful when you turn the screen off or on.
- battery usage seems tot be a little worse than before
- Camera has a problem to focus, I did not experience that before.
On the upside, MIUI 12 looks faster to me.