This doesn’t happen every time though. I’ve found that the first time you open a preview, the bug is there. But if you copy the preview URL and paste it into a new tab, then refresh that tab, the bug is no longer present. But if you go back to the post and click the “Open in a new tab” preview link again, the bug occurs.
Additionally — and this is a minor aside to the bug above — there is also a related issue of the filter acf/validate_post_id being used internally to apply code that only relates to previews (and it’s within this code that the problem lies). So I think this code, if kept, should be moved to a dedicated method/filter.