Long story short: after trying a bit iodéOS I reinstalled FairphoneOS.
After installing it I noticed that:
fastboot flashing get_unlock_ability
(bootloader) get_unlock_ability: 0
OKAY [ 0.001s]
Finished. Total time: 0.001s
I didn’t want to relock the phone in this condition, therefore I activated the developer options only to find out that the OEM unlocking option is grayed out saying that the bootloader is already unlocked.
If you haven’t solved it already your easiest option is just reinstalling FPOS and not booting into Android afterwards.
The second option involves Magisk to enable the toggle in developer settings again but is only really necessary if there are no updated factory images available (which there are rn) and you have to install OTA updates before locking (to avoid triggering rollback protection).