So I upgraded a 10.12.6 machine to 10.13.1. Ran all the updates from Apple. Then tried my policy to enable FileVault 2. It asked for a reboot as the last user logged in would then have to authenticate. Once rebooted it asked for a password and notified that FV2 would be activated. When you go into the System Preferences it shows as FV turned off. Reboot the machine and it asks for the password and for you to enable FV again.
If you wipe a drive and install High Sierra from Apple I can give an offer to that machine using the SAME policy and it works as designed.
FYI: In the user profiles under the UPGRADED machine it shows filevault 2 enabled as NO for ALL users.
Funny thing is this used to work on HS machines. We upgraded to JAMF PRO 10 but I am not sure if Apple or JAMF is the issue at this point.