It appears to not be sleep/resume causing it. Maybe it has just been a coincidence that I've only noticed it after resuming from sleep, but I was just about to put the system to sleep and I just noticed the Today Pane is empty.

I believe it is the time change that's causing it as it's just past midnight. It was showing the correct date, but no upcoming events were in the pane. I clicked the Go to Today button, and the Today Pane updated to show events again.

It is doing it when the day changes at midnight. Right at midnight the Today Pane doesn't roll over to the new day, and the upcoming events disappear. It happens too when I sleep the system before midnight and wake it the next morning.