Change this:
/* set featured image to natural size */
.blog .post-thumbnail img {
width: initial;
}
To this:
/* set featured image to natural size */
.blog .post-thumbnail img,
.archive .post-thumbnail img {
width: initial;
}
I see the Related Posts also have the same issue. To fix that:
1. Copy /inc/related-posts.php from the parent theme to the same folder in your child theme.
2. In this line in related-posts.php, replace ‘thumb-medium’ with ‘medium’
<?php if ( has_post_thumbnail() ): ?>
<?php the_post_thumbnail('thumb-medium'); ?>
3. Add related-posts to the css:
/* set featured image to natural size */
.blog .post-thumbnail img,
.archive .post-thumbnail img,
.related-posts .post-thumbnail img {
width: initial;
}