@Helight here is a thread asking, I think, very much the same question as you. Installed 3.0.2 official and tried to lock bootloader.
In brief the command
fastboot flashing get_unlock_ability
does ask about unlock ability. Not lock ability, it does not check or query any SPL dates.
It does however check device state. Install pages tell us to lock directly after install. Now it could be somewhat uncertain what might have changed the device state.
The linked thread poster opted to go back to Fairphone OS, then reinstall /e/OS.
There are a number of similar cases in this 
forum search.