By this do you mean that with the phone apparently in fastboot mode, with the green START at the top you get no response from
fastboot devices
??
This would tend to define “Non functioning fastboot” which would indicate requiring reimaging by by Fairphone if you tried to lock the phone after downgrading Android SPL.
This is mentioned as
Caution: The FP5 comes with an anti-rollback feature. Google Android anti-roll back feature is supposedly a way to ensure you are running the latest software version, including the latest security patches.
If you try installing a version of /e/OS based on a security patch that is older than the one on your device, you will brick your device. Click onDetailsbelow for detailed information
… but you really do have to Click on Details below to fully understand the issue.