Skip automatic invoice creation for suborders

functions.php

<?php

add_filter( 'storeabill_woo_auto_sync_order_invoices', function( $auto_create, $order_id ) {
    if ( $order = wc_get_order( $order_id ) ) {
        if ( $order->get_parent_id() > 0 ) {
            $auto_create = false;
        }
    }

    return $auto_create;
}, 10, 2 );