After investigating what you have experienced, I found that this is an issue that can cause the summary tracking data to be unavailable for some contacts who have registered for an event, or declined an event invitation. This is something that we will be working to prioritize so that we can work on getting a fix for this in place.
For the immediate term, the best solution is to request tracking data of a specific type(Bounces, Clicks, etc) rather than requesting the summary data.
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