How to reproduce
Navigate to the Works list of your account
Select Edit Works
Select a few works and select Edit
Look on the resulting page for a place to add a series to the works.
What should happen:
Above the “Language” option, there should be a “Series” option. It should have a select menu containing your existing series. (Creating a new series is an enhancement we can consider later.) Below the select menu should be footnote: “Works will be added in the order they were created. Please edit the series to reorder the works.”
As with other fields, it should have nothing selected by default, even if one of the selected works is in a series. Selecting a series will add all works to that series according to the works' created_at dates.