Is there a way to un-merge a ticket from a group of merged tickets?
I believe this is only possible with a direct SQL update query on the backend database.
How you you have migrate configured? e.g. Delete ticket or cancelled?
Found in Setup > Tickets > Status Types - Options 'Tab'
If you don't delete the old tickets via a merge they will still be on the system, if not then you have to go into the DB and change the deleted ticket back attribute to 0