Note – This has now been resolved. I found a couple of other people who had had similar issues.
Some long term repeat appointments cause all appointments to disappear in the calendar on the device. It also hides all appointments in other ActiveSync calendars too. The solution was to delete long term repeat appointments until the culprit is found. I will be looking through the logs to try to identify the particular issue so it may be fixed permanently.
When I add a new ActiveSync account on a Blackberry Q10 the calendar appointments appear initially but as I suppose the sync completes all the calendar appointments disappear. They still are held as I get reminders but they do not show.
As the sync continues we see the months filling out:
As the sync completes the appointments all disappear in the Q10 calendar app.
The appointments must still be present as we can see from the screenshot that they appear as a reminder on the lock screen.