Backlog Manager is displaying some odd behaviour. Moving a user story out of a sprint doesn't update the Sprint field for the User Story. ie. from sprint to backburner

It will then be listed in both the original sprint and backburner.

Only moving from one sprint to another does it update.

So I can move a user story to the backburner, effectively doubling it on the UI only, then move it to another sprint. It will still be listed on the backburner.

What in the DB Schema signals Backlog Manager to represent a User Story in it's panel? Seems like it's agf__Backburner_Rank__c. Can you mark this field as "Available for the Field Set"?