I experienced a problem where imported members who belong to an imported group do not display the custom title from that group. Instead it uses the default ladder of titles.
I tried editing / saving the user, including toggling their group memberships and custom title. I also tried rebuilding the user cache. It didn't work.
Ultimately what worked is to edit / save the usergroup itself, as if there is a usergroup cache that needed updating.
edit - it is definitely something with the imported group. It doesn't matter if the user account is imported or not.