Doing a search for this indicates that for years many people are having similar issues with Preview not working with ACF fields. Nothing I’ve read resolves my problem.
I have Guttenberg disabled.
The problem is when updating an ACF field in the editor and clicking Preview, the ACF fields show the older pre-saved value in Preview. You have to save the draft and then click Preview, but what is the point of that?
Does this have anything to do with Revisions? If Revisions Support is enabled for a custom post type in CPT-UI, Previewing ACF fields doesn’t work in either Draft or Pending Review status. When Revisions Support is disabled, Preview works for Draft status but not Pending Review.
Can anyone make sense of this? This behaviour is observed on three separate WordPress installs, all updated with the latest versions of everything.