Let's start with how we initially decided to change the logic of our Owner filter for the new Rally UI:
Initial customer feedback called for the Owner filter to work the same everywhere, using the logic from our Iteration Status page.
On the Iteration Status page, the Owner filter considers the owner at the top-level work item (User Story, Defect, Defect Suite, and Test Set), as well as the owner of any associated child work items (Task, Test Cases, etc.).
Once this change to the Owner filter was released to other pages the feedback was loud and clear that the majority of our users didn't agree after all with this logic.
We reverted this change and the Owner filter now works again as follows: