Featured image doesn't show in preview

The featured image doesn’t show in the preview when a message hasn’t been published yet. As soon as you publish the exact same message, the featured image shows.

What we found in the code:
has_post_thumbnail() is failing because get_post_meta() returns empty for scheduled posts.
I don’t know why that is, suppose it is a bug.

https://wordpress.org/plugins/custom-field-template/