1. Replace the suggested values for the Assigned To field with an explicit list of strings for the display names of your team members. This works for small teams, but becomes a maintenance problem for larger teams.
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2. Edit the .RDL reports to use account names instead of display names. Then both work items and reports match. This only works if your organization is used to account names.
Neither solution is ideal, but there pretty much the best options until the final release. Hope that helps.