function wcs_do_not_reduce_onhold_stock( $reduce_stock, $order ) {
if ( $order->has_status( 'on-hold' ) ) {
$reduce_stock = false;
}
return $reduce_stock;
}
add_filter( 'woocommerce_can_reduce_order_stock', 'wcs_do_not_reduce_onhold_stock', 10, 2 );

else if ( $order->has_status( 'processing' ) || $order->has_status( 'completed' ) ) {
$reduce_stock = true;
}

add_filter( 'woocommerce_can_reduce_order_stock', 'wcs_do_not_reduce_onhold_stock', 10, 2 );
function wcs_do_not_reduce_onhold_stock( $reduce_stock, $order ) {
    if ( $order->has_status( 'on-hold' ) && $order->get_payment_method() == 'bacs' ) {
        $reduce_stock = false;
    }
    return $reduce_stock;
}

add_action( 'woocommerce_order_status_changed', 'order_stock_reduction_based_on_status', 20, 4 );
function order_stock_reduction_based_on_status( $order_id, $old_status, $new_status, $order ){
    // Only for 'processing' and 'completed' order statuses change
    if ( $new_status == 'processing' || $new_status == 'completed' ){
    $stock_reduced = get_post_meta( $order_id, '_order_stock_reduced', true );
        if( empty($stock_reduced) && $order->get_payment_method() == 'bacs' ){
            wc_reduce_stock_levels($order_id);
        }
    }
}