dont get it, how come his persist.img file went missing from persist partition.
i flashed 8.9.20 from 8.8.30 via TWRP, after that perform factory reset with TWRP... my Mi8 works fine, although there is bugs (known bugs)
i think from Android O to P, u need to do clean flash.
do factory reset via TWRP and it should boot
i did Clean flash from 8.8.30 to 8.9.20 and it works great, despite some bugs still there
Mi8, just clean flashed with 8.9.20 from 8.8.30, so far so good ;)
btw, what should be the inprovement from Android O to P.... i didnt notice anything stated in android website.