All Recurring Event instances expire when first date passes

The event has been created with a number of occurrences throughout 2015

When it is first created, it is fine and the status is 'open', and everything is fine. However, after about an hour, the status mysteriously changes to 'expired'. It turns out this was because we had an occurrence for December 2014. If I remove this, then the problem goes away

Fair enough, I can remove the expired events and reset the status to open, but that defeats the object.

Is there something we're missing?

We have the 'expire last month's events' option enabled, and 'immediate expire past events' disabled