Stop editor from adding “amp;” after every “&”

One solution is to hook into wp_insert_post_data and do some regex magic to replace all instances of & with &:

// when saving posts, replace & with &
function cc_wpse_264548_unamp( $data ) {

    $data['post_content'] = preg_replace(
        "/&/", // find '&'
        "&", // replace with '&'
        $data['post_content'] // target the 'post_content'
    );

    return $data;
}
add_filter( 'wp_insert_post_data', 'cc_wpse_264548_unamp', 20 );

You will obviously only see changes when a post is saved/updated.