Subscriptions: no new post emails for scheduled posts

### Summary

Site subscribers do not receive email notifications after a new scheduled post is automatically published.

**This issue happens for a small portion of sites. We do not see this is a wide-spread issue.**

### Steps to reproduce the issue

The only pattern we've found so far is "automatically scheduled posts". However, our developers and Happiness Engineers can not replicate the issue with this pattern.

### Workaround

For affected sites, instead of using scheduled posts, please publish posts manually.

### What's next

If we can find the exact steps to replicate this issue, we will work on it. Until then, feel free to report here.

If you're willing to share your site credentials and help us to troubleshoot this issue, please submit a ticket and refer to this GitHub issue https://jetpack.com/contact-support/

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**Internal Ref: 2957-gh-jpop-issues, which includes more examples and troubleshooting.**

The issue names no files, tests, or entry points. Begin by tracing the scheduled-post publication flow and subscriber email notification path, then establish a reproducible affected-site case; done means scheduled posts reliably send the expected new-post emails.

Written by the indexing model from the issue text.