Block editor is automatically updating existing posts

When editing an existing blog post with the block editor, once each minute the block editor will update the post (taking changes live -- as if clicking the "Update" button), rather than just auto-saving a revision.

We've narrowed it down to ACF 5.9.0 (both free and pro). We first encountered this on a client site, and then on a second live site. I've recreated the problem on a development site running WP 5.5, Twenty Twenty, and with ACF as the only active plugin.

The live sites were running 5.4.2, so this isn't just a 5.5 issue.

Step to duplicate:

1. Edit a published post in the block editor.
2. Make some changes to the content.
3. Wait ~60 seconds with the editor open.
4. When admin-ajax.php triggers, it will also update the post and the changes will go live.

If you watch the request in the Devtools Network tab, you'll see admin-ajax.php appear, and at the same time a second URL to https://domain.com/wp-json/wp/v2/posts/1234?_locale=user.

If no changes are made after that and you just let the editor sit there, admin-ajax.php will keep triggering every minute, but it won't update again. If you then make some more changes, on the next heartbeat it'll update again.

Reproduce the issue by editing a published post in the block editor and watching DevTools Network for the recurring admin-ajax.php request and the WordPress REST request to /wp-json/wp/v2/posts/1234?_locale=user. Compare the behavior on WordPress 5.4.2 or 5.5 with ACF 5.9.0 as the only active plugin. Done means heartbeat requests save revisions without publishing unsaved changes until the editor's Update action is used.