I do have a solution that works for the case of WP_POST_REVISIONS is on, and supported by the post type. But the solution should be made in ACF, so it does not feel like the best solution as of now.
Needs more testing, and I would like to find a way for preview to work for when WP_POST_REVISIONS is inactive, or the post type not supporting it.
The recommendation should anyway to allow for WP_POST_REVISIONS to be set between 3-5 nevertheless.. if now revisions GUI with ACF fields even is a thing to use 😉
AND one more thing.
AUTO_SAVE that occurs by default every 60 seconds, that also do only save non-acf fields. That is a bummer. So the preview button still is needed to be clicked, rather than having you refresh the preview-tab. But that is not “that” much of a problem.
One solution is to have a refresh button in Preview-page that opened – so that refreshes that easily though, and can perhaps also compare timestamps.. but making a content-check for complex pages to assure that the “refresh”-symbol should take a “You have new preview-data, refresh” seem to be to complex.
