"Edit the co-authors of co-authored content" permission is not respected

Access is configured like this:

1. If the administer nodes or edit co-authors of all content permission is granted, edit access is always allowed
2. If the edit co-authors of own content permission is granted and you author the node, edit access is granted
3. If the edit co-authors of co-authored content permission is granted and you co-author the node, edit access is granted

Maybe the consideration of the administer nodes permission feels unexpected to you? I did it because the description of that permission (Promote, change ownership, edit revisions, and perform other tasks across all content types) seemed to fit changing co-authors. Let me know if you feel like this does not make sense.

Sorry for the late reply.. but had you actually tested what you said?

I just retried with a fresh install (this time v1.1.1 with Drupal 10.3.6) and I confirm the issue title: "Edit the co-authors of co-authored content" permission is (still) not respected.

Detailed steps to reproduce:

1. Allow editing co-authors only to Editors & Admin:
   
2. For completeness, these are the node edit permissions (Auth user can also create articles):
   
3. Now log in as an Authenticated user and this is the /node/add:
   

He shouldn't, but he can edit the co-authors.
So your point 2 above is false:

If the edit co-authors of own content permission is granted and you author the node, edit access is granted

that permission wasn't granted!

You're right, seems like since any field access is allowed by default, node_co_authors_entity_field_access() should explicitly return forbidden instead of neutral as access result if access is not allowed.