There is an old issue, that showed up again. The get_field() returns a wrong id when previewing the post on the frontend. Here is an older ticket with a solution https://support.advancedcustomfields.com/forums/topic/preview-solution/#post-134967.
Feel free to delete this ticket, if it’s duplicated.
Update: the fix on the link I’ve posted, breaks the ACF Blocks, which are in the content. So it’s basically outdated.
So the current version of ACF (6.1.6) handles the ACF Blocks correct, but not the get_field() on the page level. The solution for now would be to use the get_post_meta() directly.