How to replicate:
1) As a user with multiple series, edit a work in a series that is not your most recent series
2) Add a chapter to that work
3) Go to your Series page (e.g., https://archiveofourown.org/users/testy/series)
What should happen:
The newly-updated series should move to the top of your series page.
What does happen:
The date in the upper right corner of the blurb updates, but the series stays stubbornly where it is, as though you had not updated it.
Adding a new work to a series will bring it to the top; it's only new chapters that don't. For testing, also try removing a chapter to see if the series moves back down.