Aloha Elliot, we noticed this behavior again in latest ACF PRO (5.9.2) using the classic editor. When repeatedly saving a draft, the draft post id will not have any ACF postmeta created (but the main post id will). So you can see the missing fields in the revision browser for the post.
The workaround mentioned above with removing
&preview=true in the preview URL works for us (because it reverts to using the main post id, not the revision id, in the get_field() calls).
I’ll continue to investigate to see if I can figure out why the revision ID is not also getting the ACF postmeta created. Cheers!
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