Contact's Cases tab has become linked to the session setting on the dashboard "Show ALL Cases with Upcoming Activities"

Go to the case dashboard and set it so that it is only showing your cases.
Pick a contact that has a case that isn't yours.
Go to their contact summary. The cases tab will indicate they have cases.
Click on the cases tab. It will say there aren't any cases.

Go back to the case dashboard and set it to show all cases.
Pick a contact that has a case that isn't yours.
Go to their contact summary. The cases tab will indicate they have cases.
Click on the cases tab. It will now show the case(s).