We had the same issue. I think it’s caused by an incomplete implementation of Discourse dropping the category column in recent versions. When it happened to me I blamed it on us using a non-release version of Discourse from Github, which obviously may contain incomplete feature implementations. Which Discourse version do you use?
For now, I have fixed it by re-adding the category column in the table content by installing the “Add Category Column” theme component:
https://github.com/tshenry/discourse-add-category-column.git