Sorry, I’ve not been clear - I’ve been saying “Closed” for a Milestone state, when I meant “Completed”
What you describe works for me - filtering by Milestone - as long as “Milestone 1” is not “Completed”.
If you “Complete” your “Milestone 1” on the Milestones tab, then that Milestone is no longer shown in the drop down filter list on the Test Cases tab - even though all of the test cases in “Milestone 1” are still in that Test Suite. There is no way to filter for those test cases by Milestone unless you open the Milestone.
For example - the Filter drop down menu with the 6.1 Milestone not marked as Complete.
Going to the Milestones tab and marking the Milestone “Completed”:
Hit Save. Go back to your Test Suite and filter by Milestone Introduced.
The “Completed” milestone (6.1) is not in the Milestone drop down filter list any longer. Why is that filter item no longer available?