Clicking the "friendly" label button labels all visible/sensed allies, with blue allies getting precise class rating/name and non-unique purple allies getting class name for their labels. That is also true when sensing allies with an Imp. Signal Interpreter, which shouldn't give precise class rating.
(Possibly related, i had other servants in line of sight when i discovered this.)
(For all i know servants might be considered a special case, and this wouldn't really be a bug, but you know, can't know till someone reports it.)