As of Friday, August 23, 2019 at approximately 7:00PM Pacific Time, the issue described has been resolved.
Any discussion post, comment, or response that was created before August 21st around 2:30 p.m. Pacific Time is currently not available within a course's discussion forum, and users may find they can't see older posts or threads.
Posts, comments, or responses to a thread that were created after August 21, 2019 around 2:30PM are displaying as normal.
As a result, some courses' forums may look empty, similar to the image below.
Maintenance work is being done to display forum activity from before that time. This article will be updated once more information is available.