Process the Paypal express payment information from external app in Magento 2

di.xml

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <type name="Magento\Paypal\Model\Express">
        <plugin name="sulaeman_magento_paypal_model_express_plugin" 
                type="Sulaeman\PaypalWebService\Plugin\Paypal\Model\Express" 
                sortOrder="99999" 
                disabled="false" />
    </type>
</config>

Sulaeman\PaypalWebService\Plugin\Paypal\Model\Express.php

<?php
/**
 * This file is part of the Sulaeman Paypal Web Service package.
 *
 * @author Sulaeman <[email protected]>
 */
namespace Sulaeman\PaypalWebService\Plugin\Paypal\Model;

use Magento\Framework\Registry;
use Magento\Framework\Model\Context;
use Magento\Paypal\Model\Express as PaypalExpress;
use Magento\Sales\Model\Order;
use Magento\Payment\Model\InfoInterface;

class Express
{
    /**
     * @var \Magento\Framework\App\State
     */
    protected $_appState;

    /**
     * @var Registry
     */
    protected $_registry;

		/**
     * @var LoggerInterface
     */
		protected $_logger;

    /**
     * @var array
     */
    private $_areas = [
        \Magento\Framework\App\Area::AREA_WEBAPI_REST, 
        \Magento\Framework\App\Area::AREA_WEBAPI_SOAP
    ];

    /**
     * @var array
     */
    private $_paymentPayload = null;

    /**
     * @param \Magento\Framework\Model\Context $context
     * @param \Magento\Framework\Registry  $registry
		 * @param \Psr\Log\LoggerInterface  $logger
     */
    public function __construct(Context $context, Registry $registry)
    {
        $this->_appState = $context->getAppState();
				$this->_logger = $context->getLogger();
        $this->_registry = $registry;
    }

		/**
     * Interceptor capture.
     *
     * @param \Magento\Paypal\Model\Express $subject
     *
     * {@inheritdoc}
     */
    public function aroundCapture(
        PaypalExpress $subject, 
        \Closure $unknown, 
				InfoInterface $payment, 
        $amount
    ) {
        if (in_array($this->_appState->getAreaCode(), $this->_areas)
         && $this->_paymentPayload != null) {
            $payment->setLastTransId($this->_paymentPayload['id']);
            $payment->setTransactionId($this->_paymentPayload['id']);

            foreach ($this->_paymentPayload as $key => $value) {
                $payment->setTransactionAdditionalInfo($key, $value);
            }

            $payment->setIsTransactionPending(false);
        }

        return $subject;
    }

    /**
     * Interceptor assignData.
     *
     * @param \Magento\Paypal\Model\Express $subject
     *
     * {@inheritdoc}
     */
    public function beforeAssignData(
        PaypalExpress $subject, 
        \Magento\Framework\DataObject $data
    ) {
        if (in_array($this->_appState->getAreaCode(), $this->_areas)) {
            $additionalData = $data->getData('additional_data');
            if ($additionalData) {
                if (isset($additionalData['paypal_express_payment_payload'])) {
                    $this->_paymentPayload = json_decode($additionalData['paypal_express_payment_payload'], true);
                }
            }
        }

        return [$data];
    }
}

I suppose your plugin function aroundCapture() has completed bypassed the parent function since \Closure is not executed. As a result, Paypal transactions cannot be captured correctly and Paypal transaction ID(s) was missing. Please fix it by return $unknown($payment, $amount); instead of return $subject;.