The transaction documentation includes the following warning:

Similarly, if a transaction has a durable timestamp later than its commit timestamp, and a checkpoint is taken while the global stable timestamp is between these points, the transaction may or may not be visible when the checkpoint is opened with a checkpoint cursor; the behavior is unspecified. Applications should avoid this situation.

daniel.gottlieb@mongodb.com pointed that if the server eventually uses checkpoint cursors for validation, they will want this behavior to be defined – preferably so those transactions are visible. 

This isn't causing any problems right now. I'm just creating the ticket to highlight and track the (potential) issue.