It appears that Boilerplate Posts with featured images only work correctly the first time it’s published to a child site.
If the same boilerplate post is published to other child sites, the featured image is not applied. The image appears in the Media Library, but there is no database record on the child site for the featured image association.
Also, on the first child site, the Media asset is duplicated and the post is updated to use that duplicated asset filename.
This is a big content management issue.