When a forum discussion is moved from an visible forum, to a separate forum, the post is not given a groupid.
Adrian has created a solution (http://tracker.moodle.org/browse/MDL-18128) that allows the teacher/admin to assign a groupid, but it would be nice to also have the groupid automatically assigned to that of the original poster (where and when it is possible to definitively determine this groupid) as part of the move process.
From my discussion with Sam on the issue:
When a post created by student1 is moved, the teacher/admin can set the group to any group, but if student1 isn't in that group they can no longer see the responses on their discussion,
If the teacher isn't aware of the student's group, they can assign the discussion to one the student is not part of. If the system does it automatically, the teacher/admin can always over-ride it later if needed.