I know this is old, but I've just come across the same problem. The solution was as @Dave Littlejohn said, there's a missing record in that table. I checked my prod instance (this was in my dev) and there record was there in prod. Like others have said I wasn't able to add a record to the table via a New form. Therefore I exported the record from prod and imported it into the table in Dev. Record was there, comments where again highlighted as expected..