Thanks for the additional information.
It looks like the Avada theme does not support populating images from the custom field. You can try to contact Avada theme to ask for supporting this case.
Or use a tricky way, remove the post type support
Thumbnail, then change the field ID
single_image_asdf to this ID
_thumbnail_id. You can still select the Featured image in the Avada dropdown but it will show the image of the custom field.