I meant to add it to the get_children_to_save() function on the Answer children itself, instead of putting it on the LightChildren element https://github.com/edx-solutions/xblock-mentoring/pull/23/files#diff-23a9d1f1d9cda06ac8fa67c8c2f4589dR333

But that might actually not be the real solution here - cf my last comment.