After upgrading from ST 14.x to the latest (at the time) v100.7005, we one of the issues we have experienced is that the KB in the user portal wasn't listing any of the existing articles.

After tinkering around I found that is I manually open and save an article, it appears on the KB list.

After editing a few by hand I figured that the issue is the upgrade script apparently missed (a new?) requirement to have a language assigned to a KB post, otherwise it would not show up on the KB list. In fact, those saved manually were showing "English" for language and the ones I have not touched were showing "0 languages" in the KB section of the Management Portal.

So, the workaround for smaller KBs would be to open and save the articles. 

In our case, I have written a simple SQL query to add all missing articles to the st_KBArticleLanguages table - but WARNING - you better be sure you know what you are doing if you use this SQL query workaround as it may break your ST setup if you are not careful!.