Great, thanks for the explanation and passing this forward! I’ll wait patiently 
I did hear back that the devs do think this is an issue of “Rollback Protection”. There is always about a month delay for the security updates to trickle through: don’t take this as negligence on our part, instead it is just the independent timing of AOSP releases, any Vendor releases, and LineageOS releases that all need to be in place at the time of the iodéOS builds. Usually our “stable” releases are the last beta pre-release which is about a week (typically) before it is promoted to stable.
So if you could see the Android Security Update date / Vendor Security Patch date from Fairphone OS before you install iodéOS you may see it is newer than the Android Security Update of the newest iodéOS build.
You can wait for our next iodéOS OTA update (v7.3 in this case) which should then have security patch dates matching (or superseding) the prior system’s patch date. After this you should be able to finally re-lock again (but locking or unlocking will wipe out your userdata).
Alternatively, if you could force re-flash back to the prior Fairphone OS build (so not their newest one, instead the FP5-VT2I-factory.zip version, that may then be able to be “upgraded” to iodéOS 7.2 and subsequently relocked. Make sure you ensure “Allow OEM Unlocking” remains enabled in Developer Options before reflashing iodéOS!!!
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Thank for the response once again! That could be it. I had a hard time finding the info on which security patch 7.2 was on, but assumed that it’s the same as the current FPOS, as it’s on January as well. Having the info available would be very useful for avoiding these situations. Anyways I’ll wait for the 7.3 and report back on how it went!
Happy to report that I was able to relock the bootloader now on 7.3 
Thanks for the help!
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