We have faced the same issue some time back and it's the issue with the secure token on high sierra 10.13.4 and above.
Yeah welcome to the not-so-fun club. Its fairly complex at times. But essentially here's what we do:
1) Make sure techs who are imaging machines create the same account for the first account
2)During the imaging process we have the first account turn on the SecureToken for the management account
3)When an AD user logs in, they are prompted for their password and they are also give a secure token so they can log in
The reading in the firs thread is crucial to the learning process.
An alternative approach if I may be so bold :-) :
Consider why you might want your local admin account to be able to unlock the disk?
Jamf will escrow the FileVault recovery key and you can use that to gain access if needed.
@neil.martin83
A good suggestion. But its mostly so a tech can log into the machine to resolve any issues desk side if needed.