Steps to replicate the issue (include links if applicable):

• Make sure the auto-subscribe preference is enabled in your editing preferences
• Enter the 2017 wikitext editor by clicking "edit" on a talk page (not by opening that link in a new tab -- we want the load-in-place behavior)
• Add a new topic to the page and publish
• Observe the "subscribe" button by your new topic

What happens?:

The button says "subscribe", implying that you are not yet subscribed. If you reload the page, this will change to show "unsubscribe".

What should have happened instead?:

The button should have immediately shown "unsubscribe".

Other information (browser name/version, screenshots, etc.):

I suspect this is because the auto-sub is handled in a post-save hook (it's deep in the Echo notification hooks stack), and thus the subscription is actually processed after the API save is done and is provided with the new rendered content that it live-replaces the page with.

This would also happen to VisualEditor, if you found a talk page to use it on.