Rollback deletes updated entities, instead of restoring the old revision.

I used the following trick to let migrate update existing entities, instead of creating new ones:
http://drupal.stackexchange.com/a/206986/2974

So: If a product with the same sku exists (e.g if it was manually created before the migration), this will be updated, and no new product will be created.

I make sure a new revision is created with this update. (I don't remember if this happens by default or it needs some configuration..)

The problem:
On rollback, the entity is deleted, and all revisions are gone.

Instead, the old revision should be restored.