Having this issue as well. Re-adding the legend doesn't do anything and click and drag to re-adding the items doesn't work. The legend is simply stuck with nothing in it. EDIT: Got it working by removing the "stuck" legend. Then DELETING the map frame that it didn't want to show anything from. Then undoing the delete (Ctrl+Z). Then re-adding the legend now all of a sudden has all of the legend items!
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