How to Utilize Milestones Metrics like Burndown Chart if Issues Closed at End of Sprint

Our current process is that issues are closed once they have been released to production and passed a final regression test. This leads to all of our issues (unless there is a bug) being closed on the same day for a given sprint (we organize our sprints by Milestones in Gitlab).

Is there a better process we should be using so that we have a more accurate burn down chart but do not lose sight of issues that still need to go through QA?

Any help would be greatly appreciated! Thanks!