You have to downgrade to be able to use custom ROMs. The ROMs require Android 11 because apparently OnePlus posted incomplete/unusable sources for Android 12.
I don't know if the downgrade package re-locks the bootloader so I would just unlock the bootloader after you're back on OOS11. I had issues trying to unlock the bootloader after doing the OTA to the latest OOS11 so I personally would do it in this order:
1) downgrade to OOS11
2) unlock bootloader
3) OTA to latest OOS11
4) flash custom ROM
This still doesn't work for me. I have downgraded to A11 and still get the error "FAILED (remote: 'Command not supported in default implementation')" The device shows up when I type fastboot devices but continues to return this error when attempting fastboot oem unlock. How do I fix this?