Did some testing, I thought the srcset might be added when the image was inserted into the editor, but it appears that it’s not added until the content is displayed on the front end.
I’m assuming that the pluging modifies the content when
the_content hook runs. ACF does not cause this hook to run and instead uses
Try adding the following into your functions php file, this is copied from the responsive images plugin.
add_filter('acf_the_content', 'tevkori_filter_content_images', 5, 1 );