Redirect basic page to external url when visiting page
this is a cache problem. You need to set correctly the cache tags and context for the entity and it will work.
You need to do something like this to skip the cache:

  '#cache' =&gt; ['max-age' =&gt; 0],


you can find more info here: https://www.drupal.org/developing/api/8/cache/tags

Notes:


This needs to be put on hook_entity_view_alter() on the entity in your case the node, or if you want in just one field for which you want to set it. This is a work around for your redirect issue!
Try to add it to the $entity, if it doesn't work you need to replace the default '#cache' or you can add it to the $build array, this is case dependent.