Only pixels let you relock the bootloader after flashing.
Frustratingly, grapheneos only supports pixels because of that.
this post was submitted on 08 Apr 2026
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My experience with the FP4 is you can only lock the bootloader with a couple of "official" distro's. And even then there are conditions where this can go bad.
I can use the default factory version provided by fairphone, I can use e/os.
While I can install and use postmarketos or a lineage image, I can't lock the bootloader without turning it into a brick. I believe it is to do with boot partition signing by the CPU manufacturer and the android bootloader checking it.
i think it's simpler than that. It merely stores the latest security update you had when you locked your bootloader and doesn't allow you to rollback to an earlier version,but anything newer - regardless of the ROM being official or eOS/iodΓ© , would allow you to relock it.
TLDR: flashing ROM security patch version > your existing security patch version.