This was definitely an odd one. Suggested topics are serialised along with the ‘last page’ of posts in a topic. If the last post was from an ignored user, then there was a bug which meant the server thought there was another page, even though that page would be empty. Should be fixed by
https://github.com/discourse/discourse/commit/be1510b09d5341a97d8921adddefe138f6c1b524