Save User Fields from checkout when a Pay By Check pending order is created.

my_pmpropbc_added_order_save_fields.php

<?php // copy from below.
/**
 * Save User Field values from checkout when a Pay By Check pending order is created.
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
function my_pmpropbc_added_order_save_fields( $morder ) {
	// Check orders only.
	if ( 'check' !== $morder->gateway ) {
		return;
	}

	// Bail if order is added via admin dashboard.
	if ( is_admin() ) {
		return;
	}

	// Loop through all the field groups.
	$field_groups               = PMPro_Field_Group::get_all();
	$user_creation_field_groups = pmpro_get_user_creation_field_groups();
	foreach ( $field_groups as $group_name => $group ) {
		if ( in_array( $group_name, $user_creation_field_groups ) ) {
			continue;
		}

		// Save the fields.
		$group->save_fields(
			array(
				'user_id' => $user_id,
				'scope'   => 'checkout',
			)
		);
	}
}
add_action( 'pmpro_added_order', 'my_pmpropbc_added_order_save_fields', 20, 1 );