Was carried over from iOS.
A way to bypass it _should_ be possible, but will entail having the System volume of the volume group to have different properties than the Data part.
Otherwise the OS will fail to load. (on Apple Silicon Macs, macOS is fully booted already when you input the password, so if you encrypt macOS...)
On older Macs, a Preboot UEFI application application prompts you for the password prior to booting.
What you can do as a workaround:
Create a second account which you'll only use to unlock the drive and then run sudo fdesetup add -usertoadd unlockUser and then sudo fdesetup remove -user PrimaryUser. That'll give the rights to unlock the drive only to that unlock user.
You can also use sudo fdesetup removerecovery -personal to destroy the ability of the recovery key to unlock the drive.
What is the privacy implications of two users (both with administrator accounts) sharing an Apple Silicon Mac?
Both users have access to all the data in that case. It got carried over from iOS which didn't have multi-user support.
(and this is by-design, protection granularity is the volume)
Is it possible to make sure that encryption key only available using this "unlock" user passphrase?
Also in case of travel or emergency it's much easier to just power it off. At the same time there is tons of ways how someone can steal your day-to-day lock screen password.
I obviously find it being absolutely terrible "design" decision since there no way on earth anyone can count disk encryption key that is unlockable by user password or faceid secure.
PS: If someone have any idea how having separate boot password can be hacked aroud I'll really appreciate the advice.