Mi A2 bootloader relock
It is not possible to relock the bootloader successfully on latest firmware since the official Pie update was released (V10.0.1.0.PDIMIFJ). The phone shows the yellow screen but then it reboots and keeps bootlooping. Last known bootloader that allows relocking with custom OS installed is the one from Oreo MIUI (V18.104.22.168.ODIMIFE).
What I tried so far:
Tested MIUI V22.214.171.124.ODIMIFE bootloader/firmware, successfully relocked.
Tested all Pie updates up to MIUI V10.0.17.0.PDIMIXM bootloader/firmware, device no longer boots when relocked.
Mi A2 supports verified boot 1.0, so I ensured everything is set up correctly device side and enabled verity which was previously disabled, including properly baking the key into the kernel. I also tried signing the builds with test keys and custom keys. Oreo bootloader still worked fine, but Pie bootloader still didn't boot.
It's pretty certain that Xiaomi changed something in the Pie bootloader (not sure if it was intentional) and our only hope is that it will get fixed with the upcoming Android 10 update. Unfortunately we don't have any contact in Xiaomi yet to try and resolve this issue.
Edit: The bootloader from Android 10 update that was released (MIUI V126.96.36.199.QDIMIXM) is also broken in the same way as Pie bootloader.