Issue #3506667: FieldMapperBase not supporting programmatically declared properties

src/FieldMapper/FieldMapperBase.php

@@ -295,20 +295,7 @@ abstract class FieldMapperBase extends PluginBase implements FieldMapperInterfac
         return NULL;
       }
 
-      $derived_entity_type = $this
-        ->getExternalEntityType()
-        ->getDerivedEntityType();
-      if (empty($derived_entity_type)) {
-        $this->logger->warning('FieldMapperBase::getFieldDefinition(): Cannot get external entity derived type.');
-        return NULL;
-      }
-
-      $fields = $this
-        ->entityFieldManager
-        ->getFieldDefinitions(
-          $derived_entity_type->id(),
-          $derived_entity_type->id()
-        );
+      $fields = $this->getExternalEntityType()->getMappableFields();
 
       $this->fieldDefinition = $fields[$this->fieldName] ?? NULL;
     }