Change the upload directory of Advanced Custom Fields file upload field.

acf-change-file-upload-directory.php

<?php
// ACF upload prefilter
function gist_acf_upload_dir_prefilter($errors, $file, $field) {
    
    // Only allow editors and admins, change capability as you see fit
    if( !current_user_can('edit_pages') ) {
        $errors[] = 'Only Editors and Administrators may upload attachments';
    }
    
    // This filter changes directory just for item being uploaded
    add_filter('upload_dir', 'gist_acf_upload_dir');
    
}

// ACF hook, set to field key of your file upload field
add_filter('acf/upload_prefilter/key=field_580ac5acd5616', 'gist_acf_upload_dir_prefilter');

// Custom upload directory
function gist_acf_upload_dir($param) {
    
    // Set to whatever directory you want the ACF file field to upload to
    $custom_dir = '/uploads/custom_dir';
    $param['path'] = WP_CONTENT_DIR . $custom_dir;
    $param['url'] = WP_CONTENT_URL . $custom_dir;

    return $param;
    
}

getting this error:
Unexpected response from the server. The file may have been uploaded successfully. Check in the Media Library or reload the page.

need to change ln 19 to...
add_filter('acf/upload_prefilter/key=field_580ac5acd5616', 'gist_acf_upload_dir_prefilter', 10, 3 );

Thanks for the gist, it's really helped me from banging my head against the wall. Is it possible to also further sort uploads by user id in addition to the custom directory?

is deleting the files working for you?