1) Pay attention to the current sort order for the layout's underlying table. Your layout depends on a specific sort order inorder to provide you with the grouped records you expect to see. Any action that re-sorts the data, and this could happen on any layout that uses the same underlying table, may change the order and grouping of your records.
2) Perhaps you have some blank records and a an ascending sort has grouped them all at the top of your found set. This would only be visible in browse not layout mode as you describe.