This is working for me, it updates the last modified author when editing from the front end.
Just add this to your functions file.
function user_acf_save_post( $post_id )
update_post_meta( $post_id, '_edit_last', get_current_user_id() );//update last modified by
// run after save post
add_action('save_post', 'user_acf_save_post', 10);
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