Hello Andrew,
Sorry for later reply to this,
After debugging long time ago, it seems like your database seems like get corrupted. I'm not really sure what factor causing this issues, maybe some of your plugin causing this.
What we fix this is go to your
backend > admin tools > Repair & Optimise Tables then everything is work fine now even your backend post listing page also can show out the discussion post now.
May i know do you have any 3rd party component handle the database?
Regarding with this issues ->
http://awesomescreenshot.com/0db3q97afb :
>> I have tested in your site, it seems like I cant reproduce on this, Please advise.