Endpoints such as (POST profile-posts/{id}/), (POST profile-post-comments/{id}/) have a property author_alert_reason. In all cases, which I found in xenForo source code, it has a string type.
Thank you for reporting this issue, it has now been resolved. We are aiming to include any changes that have been made in a future XF release (2.2.14).
Change log:
Fix profile post/comment author alert reason API documentation