I just tested that on a test install, changed to Choice, added a few choices.. sure enough, nothing. I hacked at it for a bit, but can't figure it out, looks like the subject is stored in a different table to the form, and yet, when viewing a single ticket, the $ticket instance has access to the correct format.. so, a hacky/slow/shite implementation that will get you over the hump, assuming you're ok with the perf hit, /include/staff/tickets.inc.php after line 526:if($subject_field instanceof ChoiceField){//HACK.. likely incredibly slow
$subject = Ticket:($T)->getSubject();
}