To fix this issue, we need to create ‘controller.php’ file at the below location.

appcodeMagemonkeysCustomerControllerIndex

<?php
Namespace MagemonkeysCustomerControllerIndex;

class Controller extends MagentoFrameworkAppActionAction
{

protected $customer;

protected $customerFactory;

public function __construct(
    MagentoCustomerModelCustomer $customer
    MagentoCustomerModelResourceModelCustomerFactory $customerFactory
)
{
    $this->customer = $customer;
    $this->customerFactory = $customerFactory;
}

public function execute()
{

    $custom = $this->_customerFactory ->create();
    $custom = $custom->setWebsiteId($helperData->getWebsiteId());
    $custom = $custom->loadByEmail("jayesh@webtechsystem.com");
    $custom->setCustomAttributeCode(custom attribute value);   

  // Main code to resolve this issue.
    $customerData = $custom->getDataModel();
    $customerData->setCustomAttribute('custom_attribute_code', 'custom attribute value');
    $custom->updateData($customerData);

    $custom->save();
}
}