Release Burnup and Release Cumulative Flow reports are not including data after changing the Release start date to an earlier date.
This is working as designed since the report aggregation records the Release or Iteration OID at the time of the nightly data collection. Changing the Release start date would not result in changes to the the artifact historical data or OID. Running a data or report data repair will not help.
As a workaround the customer could tag all the top level User Stories in the report and run the Tagged Story Burndown report/chart. While this would not provide the same report, it would generate the historic data since the report runs off of the tags which currently exist on those artifacts.