reverted:
--- b/core/modules/migrate/src/Audit/IdAuditor.php
+++ a/core/modules/migrate/src/Audit/IdAuditor.php
@@ -50,7 +50,7 @@
     $conflicts = [];
 
     foreach ($migrations as $migration) {
+      $migration_id = $migration->getPluginId();
-      $migration_id = $migration->getBaseId();
       $conflicts[$migration_id] = $this->audit($migration);
     }
     ksort($conflicts);
only in patch2:
unchanged:
--- a/core/modules/migrate/src/Plugin/migrate/id_map/Sql.php
+++ b/core/modules/migrate/src/Plugin/migrate/id_map/Sql.php
@@ -981,7 +981,7 @@ public function getHighestId() {
       $migration_manager = $this->getMigrationPluginManager();
       $migrations = $migration_manager->getDefinitions();
       foreach ($migrations as $migration_id => $migration) {
-        if ($migration['id'] === $base_id) {
+        if ($base_id === substr($migration_id, 0, strpos($migration_id, $this::DERIVATIVE_SEPARATOR))) {
           // Get this derived migration's mapping table and add it to the list
           // of mapping tables to look in for the highest ID.
           $stub = $migration_manager->createInstance($migration_id);