参照
wordpress.org
A new default intermediate size, medium_large, has been added to better take advantage of responsive image support. The new size is 768px wide by default, with no height limit, and can be used like any other size available in WordPress. As it is a standard size, it will only be generated when new images are uploaded or sizes are regenerated with third party plugins.
The medium_large size is not included in the UI when selecting an image to insert in posts, nor are we including UI to change the image size from the media settings page. However, developers can modify the width of this new size using the update_option() function, similar to any other default image size.
ということらしいので、
update_option( 'medium_large_size_w', 0 );
をfunctions.phpに追記して、解決しました。
閲覧頂いた方ありがとうございました。