So, if I am following this correctly, the product owner is accepting the user stories at the iteration review, and that review falls after the sprint is over, rather than being on the last day of the sprint, is that right?
Ideally, the product owner should be reviewing and accepting user stories during the sprint, rather than after. Even more ideally, the PO is accepting the work as it gets completed, rather than waiting until the end of the sprint. Here's why - if the PO reviews a story and it is not acceptable, if you're still in the sprint, the team can then go back and fix the issue and still get the work done during the sprint as planned. If you wait until the end, and the story isn't ready to be accepted, you now have unfinished work to deal with scheduling into the next iteration. Your burndown report is correct, as it shows work accepted DURING the iteration, which has not happened in this case.