If an order is placed On Hold, change/keep the subscription's status as Active.

wcs_sub_active_order_onhold.php

<?php

/*
 * If an order is created and placed On Hold, keep the subscription Active
 * If an order is changed to On Hold, change the subscription status to Active
 *
 * wcs_order_contains_subscription( $order, 'any' ) | any / parent / renewal / switch
 */

add_action( 'woocommerce_order_status_changed', 'wcs_sub_active_order_onhold', 10, 4 );

function wcs_sub_active_order_onhold( $order_id, $status_from, $status_to, $order ) {
    if($status_to === 'on-hold') {
        if ( wcs_order_contains_subscription( $order, 'any' ) ) {
            $subscriptions = wcs_get_subscriptions_for_order( $order, array( 'order_type' => array( 'any' ) ) );
            foreach( $subscriptions as $subscription_id => $subscription ){
                $subscription->update_status( 'active' );
            }
        }
    }
}