For anyone else that runs into this issue. It appears to be a bug. I have been able to replicate it on two sites and will test it on more sites soon. It happens under the following conditions.
If a post with ACF fields is scheduled, and the scheduled post is then edited and any content is changed in the fields OR the main content area, whether the post is saved or not before clicking the preview button at the top of the window and selecting “Preview in new tab”, any custom field content will be gone in the preview. If you go directly from the preview window back to the All Posts page and preview the post from there the content in the fields will be gone. However, if you left the original edit tab open the fields will be there and you can re-save the post. If you then click the “View Post” link that briefly appears at the bottom of the window, the field content will be there.
