This tutorial is a solution if your store switcher is not working when customer is logged in Magento.

It happens because there is a difference between 2.3-develop and 2.3.3 for the di.xml file of the store module.

Check the following changes to resolve this issue :

First of all override the following file :

magento2/app/code/Magento/Store/etc/di.xml

& Replace the following code : 

 <type name="MagentoStoreModelStoreSwitcher"> 
     <arguments> 
         <argument name="storeSwitchers" xsi:type="array"> 
             <item name="cleanTargetUrl" xsi:type="object">MagentoStoreModelStoreSwitcherCleanTargetUrl</item> 
             <item name="manageStoreCookie" xsi:type="object">MagentoStoreModelStoreSwitcherManageStoreCookie</item> 
             <item name="managePrivateContent" xsi:type="object">MagentoStoreModelStoreSwitcherManagePrivateContent</item> 
             <item name="hashGenerator" xsi:type="object">MagentoStoreModelStoreSwitcherHashGenerator</item> 
         </argument> 
     </arguments> 
 </type>

With 

 <type name="MagentoStoreModelStoreSwitcher"> 
     <arguments> 
         <argument name="storeSwitchers" xsi:type="array"> 
             <item name="cleanTargetUrl" xsi:type="object">MagentoStoreModelStoreSwitcherCleanTargetUrl</item> 
             <item name="manageStoreCookie" xsi:type="object">MagentoStoreModelStoreSwitcherManageStoreCookie</item> 
             <item name="managePrivateContent" xsi:type="object">MagentoStoreModelStoreSwitcherManagePrivateContent</item> 
         </argument> 
     </arguments> 
 </type>

We deleted the following line and the store switcher started working for logged in customers.

<item name="hashGenerator" xsi:type="object">MagentoStoreModelStoreSwitcherHashGenerator</item>

Please let us know if this article is helpful to you or not.