Thanks for reporting this. I’m seeing two separate issues. The first issue is that hidden posts are being returned by the TopicViewWordpressSerializer. We need to either stop returning hidden posts, or add the hidden property to the data that is returned from Discourse. This would allow the plugin to handle hidden posts.
The second issue is that the list of topic participants that is returned by Discourse is set based on which participants the user making the request can see. Since the plugin is requesting Discourse topics as an admin user, staff users who have created whisper posts in the topic are being returned in the participants list. I’m going to have to look into this some more.