Why do our group forums keep disappearing?

We have a handful of private access groups in our community that contain the forum app, documents app and some the Calendar app.

We've noticed since our launch that the group forum thread list keeps disappearing intermittently and, even though the forum is full of threads, the landing page for the forum says that there are 'no post to show. Be the first to ask a question' 

They will reappear eventually but we can't work out why they're disappearing.

At first I thought it was just some quirk for me as my colleague could view the list but I couldn't but it's now being reported by other members of our community too.

What's happening?