There is no complete solution possible. And that is not new.
Any image can be a part of a gallery in a post that is not that image’s parent. So the post parent is not a useful parameter. The new interface makes such galleries easier to create, but they were possible in earlier versions too.
Even if the attachment’s parent contains a gallery with that image – there could be another gallery with the same image. Yes, you can create multiple galleries and re-use images.
What you can do:
- Find all posts with galleries.
- Find all galleries using that image.
- Parse those galleries to detect the order (good luck with random!).
- Try to detect from which post the user came to see the current image (sessions?). Maybe she found it per Google.
Alternative:
Create an endpoint for post galleries to keep the relation intact. With an URL like …
example.com/blog/postname/gallery/1/4/
… you would know you are on the fourth attachment for the first gallery that belongs to the post with the slug postname.