Can you confirm for certain the exact Android SPL of this build?
If you performed the preinstall unlock on this build then that date is the one locked into your phone’s anti-rollback index.
You are correct that Vendor SPL is not part of the “can I relock” comparison.
This thread Installed 3.0.2 official and tried to lock bootloader explores an issue affecting users who come back to relocking the bootloader after a gap (like your situation).