Steps to reproduce:
You need three user accounts: User1, User2, and User3. (You don't actually need access to User3's account, but the account needs to exist.)
Log in as User2.
View your profile, go to Manage Pseuds, and add a new pseud with the name User3.
Log in as User1.
Create a work with a brand new series, or add a new series to an existing work.
Click on the series name at the bottom of the work.
Click "Edit Series"
Check "Add co-creators?" and enter "User3" in the text field that appears.
No one is added as a series co-creator. The byline stays the same.
You can work around this bug by either editing the work to add the co-creator (which should automatically add them to the series as well), or by entering "User3 (User3)" instead of just "User3."
What should happen:
Either you should be presented with a co-creator disambiguation form (like the ones for Works and Chapters), or you should be able to add User3 as a co-creator just by entering their username.