medium_large image size not displaying properly on the frontend?

I saw this problem recently on a project, and the answer is to disable responsive image markup. Simply add the below code into your theme’s functions.php and you are golden:

add_filter( 'wp_calculate_image_srcset', '__return_false' );

Ref: How do I disable responsive images in WP 4.4?

Afterwards, you can insert the Medium Large a.k.a. medium_large images (if enabled) into your post editor and they will load correctly on the frontend.

For the record I think the Medium Large size is very underrated, and saves tons of time. We use it by default in our free HoverCraft theme for featured images, and also the default size for inserting media too. This is a special image size in WordPress because the width is fixed at 768px wide with auto-height depending on the ratio of the image file you uploaded, which makes theming and layout very easy to automate.