Burndown chart displaying inaccurate number of open issues

Zendesk ticket (internal link): https://gitlab.zendesk.com/agent/tickets/92157

A customer has a milestone in which all the issues have been closed. When this finished, the Burndown chart doesn't seem to recognize that all issues have been closed:

As you can see, the chart shows that there are 5 open issues whereas the right sidebar shows that there are 0 open issues.

The customer is running 10.5.3-ee and has cleared the cache to no avail. He has also confirmed that no issues have been reopened, and that the issues were closed days before the milestone due date.