I am using hard-crop on featured images like this :
add_theme_support( 'post-thumbnails' ); set_post_thumbnail_size ( 635, 200, true );
I have no other plugin installed, not even Jetpack. No matter how large the image I upload wordpress won't add srcset for my cropped featured images.
EDIT: The problem seems to be with hard crop feature because the srcset shows correctly on any other image posted in article (from media for example). If I deactivate the hard crop the srcset shows correctly on the thumbnails too.