Can get shipping extension_attributes in observer Magento 2

Hi I want to save a shipping address attribute by observer sales_model_service_quote_submit_before
Bellow My observer:

  public function execute(Observer $observer)
        {

            $quote = $observer->getEvent()->getQuote();
            $fields = $this->fieldsManagement->getByQuoteId($quote->getId());


            $objectManager = MagentoFrameworkAppObjectManager::getInstance(); 
            $resource = $objectManager->get('MagentoFrameworkAppResourceConnection');
            $customer_res = $objectManager->create('MagentoCustomerApiAddressRepositoryInterface');
            $customer = $customer_res->getById(101);
            $shippingAddress = $quote->getShippingAddress();
            $shippingAddressExtensionAttributes = $shippingAddress->getExtensionAttributes();
            if ($shippingAddressExtensionAttributes) {
                $customField = $shippingAddressExtensionAttributes->getDistance();
$customer->setCustomAttribute('distance',$customField);
            }else{
                $customer->setCustomAttribute('distance','not save');
            }   
        }

It Is the observer, I have the
requirejs-config.js

var config = {
    config: {
        mixins: {
            'Magento_Checkout/js/action/set-shipping-information': {
                'Danielozano_CustomerAttribute/js/action/set-shipping-information-mixin': true
            }
        }
    }
};

set-shipping-information-mixin.js

define([
    'jquery',
    'mage/utils/wrapper',
    'Magento_Checkout/js/model/quote'
], function ($, wrapper, quote) {
    'use strict';
    alert('test');
    return function (setShippingInformationAction) {

        return wrapper.wrap(setShippingInformationAction, function (originalAction) {
            var shippingAddress = quote.shippingAddress();
            if (shippingAddress['extension_attributes'] === undefined) {
                shippingAddress['extension_attributes'] = {};
            }

            shippingAddress['extension_attributes']['distance'] = 'tested';
            //shippingAddress['extension_attributes']['distance'] = jQuery('[custom_attributes[distance]]').val();

            return originalAction();
        });
    };
});

Alos have extension_attributes value to please check bellow image:
https://i.stack.imgur.com/rMwme.png

extension_attributes.xml

< ?xml version="1.0" encoding="UTF-8"?>

    
        
    
    
        
    
    
        
    
    
        
    
    
        
    

fieldset.xml

di.xml

ShippingAddressManagement.php

< ?php
namespace DanielozanoCustomerAttributePluginMagentoQuoteModel;

class ShippingAddressManagement
{
    protected $logger;

    public function __construct(
        PsrLogLoggerInterface $logger
    ) {
        $this->logger = $logger;
    }

    public function beforeAssign(
        MagentoQuoteModelShippingAddressManagement $subject,
        $cartId,
        MagentoQuoteApiDataAddressInterface $address
    ) {
        //exit;
        $extAttributes = $address->getExtensionAttributes();       
        if (!empty($extAttributes)) {
            try {
                $address->setDistance($extAttributes->getDistance());

            } catch (Exception $e) {
                $this->logger->critical($e->getMessage());
            }
        }
    }
}

My issue is when I place order address save but on distance attbiute it should save the distance value test but it show blank. If I use $shippingAddress->getExtensionAttributes() to save as a value then it show 1 on distance field. So 1 is save.