Steps to reproduce

You'll need access to two accounts for this. To make this easier to demonstrate, these steps assume the second account has a work and the work is old enough that subscriptions have already gone out for it.

1. Log in to your first account

2. Hi, username! > My Dashboard

3. Press "Subscribe" to subscribe to yourself

4. Follow "Preferences" link

5. Check "Allow others to invite me to be a co-creator."

6. Press "Update"

7. Log out

8. Log in to your second account

9. Hi, username! > My Works

10. On one of your works, follow the “Edit” link

11. On the edit page, follow the “Add Chapter” link

12. Fill in required fields

13. Press "Post"

14. Quick like a bunny, follow the "Add Chapter" link on the top of the work

15. List your first account as a co-creator for this chapter

16. Fill in required fields

17. Press "Post"

18. Again, quickly, log out

19. Log in to the first account

20. Check your email for your first account

21. Use the link to accept the creator invitation

22. Now wait for the subscription email

What happens

This is what the subscription email looks like:

sarken and nivens posted a new chapter of Multi-Chapter Co-Created Work (981 words):

Chapter 2: chapter with single author (56 words)
by sarken and nivens

Chapter Summary:
Flying solo.

Chapters: 3/?
Fandom: Battlestar Galactica (2003) and BSG 2003
Rating: General Audiences
Warnings: No Archive Warnings Apply
Characters: Number Six (Battlestar Galactica 2003)

Summary:

Resurrection.

–

sarken and nivens posted a new chapter of Multi-Chapter Co-Created Work (981 words):

Chapter 3: co-created chapter (67 words)

Chapter Summary:
Tag team!

But that's not right! Chapter 2 is just by sarken. Chapter 3 is by sarken and nivens.

What should happen

The byline under the chapter title should be the chapter byline, not the work byline.