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 and with "User3" listed as a co-creator.
Because "User3" is ambiguous, you will be prompted to clarify whether you meant "User3 (User3)" or "User3 (User2)." Choose one – it doesn't matter which – and finish posting.
The posted work does not belong to any series.
What should happen:
The posted work should belong to a series.
(Note that this is not identical to AO3-5606, which deals with adding ambiguous co-creators directly to the series.)