Fix acf-json for Roots Sage 9

setup.php

<?php

/**
 * Fix location of ACF local JSON.
 *
 * Since Sage does some surgery on the WordPress template locations, ACF looks in
 * the wrong location for the acf-json directory. We will fix this by manually
 * hooking into that functionality and attempting to save in the right spot.
 *
 * @param  string  $path
 * @return string
 */
add_filter('acf/settings/save_json', function ($path) {
    $targetDir = get_template_directory().'/acf-json';

    return (file_exists($targetDir) && is_dir($targetDir)) ? $targetDir : $path;
});

Update: This is now fixed in Sage 9 Beta.