@ctw

Already tried to enable it by pushing out a configuration profile?
If yes, check other config profiles if they do block FUS.

I second @txhaflaire, I have it working on 10.14.3 with a config profile.

I was trying through scripts and just policies. I'll give this a shot and report back.

Have a look on the Mac in the profiles, see if one of them is blocking it. I found with some things the profiles were blocking things even though you cant set them in there. The iSight camera is set using Restrictions, but the Security & Privacy will block it unless you have both in the configuration together. Maybe it is the same with FUS.

I have it all set now, it was in the configuration profiles. I just wasn't able to create one because of the bug for site admins atm.

I just discovered that I am having this issue as well. I have FUS enabled on all of my profiles (because of the nature of the computers on our campus) and yet, I have a number of computers that I cannot get FUS to turn on. I am not seeing a profile that is disabling it... Any ideas where to look?

Resurfacing this if this might help;

I found that the same change only enabled switching for the first user.  However, changing my profile to the User level allowed the all other users to switch users.