Thanks for your posting. On the Milestones overview page, TestRail separates between active/upcoming (with a start date in the future) and completed milestones:
Each section is ordered in a slightly different way. Active milestones are ordered by their due date, if available, and their name otherwise. Upcoming milestone are ordered by their start date and completed milestones by their completion date, respectively. If you have multiple active milestones, you can look into assigning a due date to set the order or look into changing the name accordingly.
I hope this helps!