I’m going to keep this thread active, as the problem is still ongoing:

-o- In addition to posts disappearing, it is no longer possible to delete other people’s posts in the private forum feed.
-o- If you click “Load More” at the bottom of the feed, it will list the next 20 results and hide the previous 20. Doing this used to add 20 previous replies to the bottom of the feed, so you could see them all.
-o- Commenting to a post seems bugged in the feed. You have to open the post and reply from there for it to work.

These things started for me at the same time that the disappearing posts glitch appeared.