Advanced Custom Fields 5.9.0 - Publishing updates while editing

Version 5.9.1 of ACF has been released to fix this issue. Full changelog.

Advanced Custom Fields 5.9.0 (both free & pro versions) contains a bug causing various issues surrounding the saving and previewing of posts.

For example: 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.

They have a hotfix available and are testing it further. In the meantime, either roll back to the previous version of ACF, or avoid editing any existing posts in the Block editor, since changes will be published every ~60 seconds as you’re editing.