diff -u b/core/modules/content_translation/migrations/d6_node_master.yml b/core/modules/content_translation/migrations/d6_node_master.yml
--- b/core/modules/content_translation/migrations/d6_node_master.yml
+++ b/core/modules/content_translation/migrations/d6_node_master.yml
@@ -13,6 +13,9 @@
 process:
   # If you are using this file to build a custom migration consider removing
   # the nid and vid fields to allow incremental migrations.
+  # In D6, nodes always have a tnid, but it's zero for untranslated nodes.
+  # We normalize it to equal the nid in that case.
+  # @see \Drupal\node\Plugin\migrate\source\d6\Node::prepareRow().
   nid: tnid
   vid: vid
   langcode:
@@ -36,6 +39,10 @@
   revision_log: log
   revision_timestamp: timestamp
   content_translation_source: source_langcode
+  #  unmapped d6 fields.
+  #  translate
+  #  moderate
+  #  comment
 destination:
   plugin: entity_master:node
   translations: true
diff -u b/core/modules/content_translation/migrations/d7_node_master.yml b/core/modules/content_translation/migrations/d7_node_master.yml
--- b/core/modules/content_translation/migrations/d7_node_master.yml
+++ b/core/modules/content_translation/migrations/d7_node_master.yml
@@ -33,6 +33,9 @@
   revision_log: log
   revision_timestamp: timestamp
   content_translation_source: source_langcode
+  #  unmapped d6 fields.
+  #  translate
+  #  comment
 destination:
   plugin: entity_master:node
   translations: true
diff -u b/core/modules/migrate_drupal/src/NodeMigrateType.php b/core/modules/migrate_drupal/src/NodeMigrateType.php
--- b/core/modules/migrate_drupal/src/NodeMigrateType.php
+++ b/core/modules/migrate_drupal/src/NodeMigrateType.php
@@ -42,7 +42,7 @@
    * @internal
    *   Only to be used by migrate_drupal_migration_plugins_alter().
    */
-  public function getNodeMigrateType(array $definitions) {
+  protected static function getNodeMigrateType(array $definitions) {
     $migrate_type = static::NODE_MIGRATE_TYPE_MASTER;
     $version = '';
 
@@ -94,7 +94,8 @@
     else {
       // Check the followup migrations.
       foreach ($definitions as $key => &$definition) {
-        if ($definition['id'] == 'd7_entity_reference_translation' || $definition['id'] == 'd6_entity_reference_translation') {
+        $tags = $definition['migration_tags'] ?? NULL;
+        if (array_search('follow-up migration', array_map('strtolower', $tags))) {
           $version = substr($definition['id'], 1, 1);
         }
       }