We are seeing the same thing on v 12.5.6.2. All the data is in the database, but it seems like a link to those in the webpages are not working. You can create a new category and article and it still shows the number of articles and then blank right side. We have tried .Net fixers, reinstalling the website, and rebuilding the kbs_index folder. Nothing seems to work for now.
My next step is to do a brand new install and try to compare the tables if there is a value we are missing.