This is technically partially the Jetpack plugin’s fault, but it’s common enough I figured other users should at least get a heads-up about it:
Apparently if you forget to leave a Read More block ahead of the real content, emails sent by Jetpack will include the whole post in the email, which will be emailed to all subscribers, regardless of patreon log in or payment, of course. Definitely something to be aware of for other creators!
For the part of the Patreon plugin, might want to auto-insert the read-more block at the top or something to help posts stay hidden from non-logged in contexts?