Hi there, I was also one of the people with the problems on Mi 13 Ultra but I believe I got the phone working. (Fingers crossed)
The first thing i did wrong was installing the HyperOs 2.0 on A14 TWRP.
After doing that i had the crashes and reboots everyone else already described.
Next I flashed the new A15 TWRP and flashed the ROM again in TWRP. Didn't help, same issues.
After that I flashed the rom through fastboot (windows_install_upgrade.bat). Didn't help, same issues.
Then I started reading the forum and also made a logcat which I attach.
I started the log after the phone booted and ended it after a crash and reboot. Maybe it will help diagnose the problem.
Log
The thing that finally helped me was going to TWRP and clearing the cache partition. ( "Helped" assuming the phone will not start misbehaving again)
After doing that and booting into system I pressed the lock button (doing that previously froze the phone instantly)
It froze the phone for about 5 seconds but after that it recovered and is working as it should.
Im too scared to restart it again fearing that the issues may come back.
I hope this can help somebody. Merry Christmas