Hi guys, same issue here with unlock. Do you have your display (lcd screen) replaced? It is some kind of bug in SW, so we should hope, that they will fix it. Until that I am on MIUI 11..
I had same problem = unable to unlock phone after update, just MIUI logo shows and phone lock again. Only way was wipe all data from recovery menu (volume up + powerbutton until menu shows).
Every time i try to unlock my phone i see my own background but i cannot unlock with my fingerprint or face. When I try with my code the screen goes immediately black after I have pressed the last digit. Then the phone tries to start-up again. I've tried the reboot from the main menu (after pressing powerbutton and volume up), but with no results.
Xiaomi has started issuing MIUI 12 to the EU versions of the Redmi K20 Pro and Mi 9T Pro. The devices, which have already received the OS upgrade in other regions, have been included in Round 1 of Xiaomi's MIUI 12 release schedule. Other devices that have been upgraded to MIUI 12 in multiple regions include the Mi 9, Mi 9T and the Redmi K20.