Since the final release of Mojave 18A391, Macs that have a config profile set to defer FileVault until the next logout/restart do not prompt the user to enable FileVault when restarting/logging out of an AD-mobile user.
You can log into a local admin user and the normal/expected behavior occurs upon a restart/log out. I've seen this on both 2018 T2 and 2017 (and earlier) Macs. Anyone else? This adds manual steps that a user was able to perform on their own in the past :(
I just filed a Bug Report 44798927 for this issue.