Yes.
But that isn’t really a Fairphone issue per se, that’s what rollback protection is meant to do on a modern phone. The phone keeps track of the rollback index (security patch level in Fairphone’s case) so you / an attacker can’t install an older version which might contain unpatched known vulnerabilities that could be exploited.
Locking the bootloader enforces that check, but OEMs can also opt to always enforce it as well.
Normally that rollback index should get cleared when the phone is completely reflashed (search this forum / the Android documentation for specifics, I don’t have the links handy), and the Pixels manage to do that just fine.
Now there’s apparently a bug in certain Qualcomm bootloaders (according to the CalyxOS devs) which affects the Fairphone as well that leads to the rollback never being reset. So on any devices (not only Fairphones) affected by that the only way is forward.
I don’t think Fairphone as a company has the necessary access / ressources to fix low level Qualcomm bootloader bugs, I might be wrong, you’d have to ask them.
But they could make the install process safer by implementing checks and locking the bootloader during installation. And they failed to do that for two years, that is absolutely on them!
(Got a bit ranty again, sorry, I’ll see myself out 
)