In fact, when received, the bootloader version was still FP5.UT2E.B.078.20241105, which corresponds to Fairphone OS Android 14. As if /e/OS installation was failed…
I flashed this version, to check the phone was functional under Fairphone OS.
Then unlocked bootloader, to flash Official /e/OS (Android 13).
Since then, I kept it unlocked (because I understand from what you say that allow-flashing is not available with Android 13…)