works completely normally for me. Only if test mode is activated emails will be sent more than once.
got same error from using is 8.x-1.7:
Drupal\Core\Entity\EntityStorageException: SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'uid' cannot be null: INSERT INTO "profile" ("revision_id", "type", "uuid", "status", "uid", "is_default", "data", "created", "changed") VALUES (:db_insert_placeholder_0, :db_insert_placeholder_1, :db_insert_placeholder_2, :db_insert_placeholder_3, :db_insert_placeholder_4, :db_insert_placeholder_5, :db_insert_placeholder_6, :db_insert_placeholder_7, :db_insert_placeholder_8); Array ( [:db_insert_placeholder_0] => [:db_insert_placeholder_1] => customer [:db_insert_placeholder_2] => b4d7f19f-2ce8-4011-85ce-5b9bbfb4ea8f [:db_insert_placeholder_3] => 1 [:db_insert_placeholder_4] => [:db_insert_placeholder_5] => 0 [:db_insert_placeholder_6] => a:0:{} [:db_insert_placeholder_7] => 1702131196 [:db_insert_placeholder_8] => 1702131196 ) in Drupal\Core\Entity\Sql\SqlContentEntityStorage->save() (line 817 of /var/www/mrstea_app/web/core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorage.php).
switching to dev solve the issue.
Fulgrim → created an issue.