Admin::buildShow doesn't call configureShowFields in any attached AdminExtensions - AdminExtension contains this method though, so assume the intention was for it to call it.

Admin/Admin.php

@@ -616,6 +616,10 @@ abstract class Admin implements AdminInterface, DomainObjectInterface
         $this->configureShowField($mapper); // deprecated, use configureShowFields instead
         $this->configureShowFields($mapper);
 
+        foreach($this->getExtensions() as $extension) {
+            $extension->configureShowFields($mapper);
+        }
+
         $this->show = $collection;
     }