We do we get <Unknowns> sometimes in Wait Events drilldown for the following reasons:
1. SQL Server did not add the metric to the event because it was too difficult to get or the metric was no longer available.
2. The event was created by a process that does not have that metric. Typically these are internal SQL Server processes.
There are times when we get <Unknowns> in Wait Events drilldown is unavoidable based on above reasons.
In 11.5, we just add a note in help to explaining why we get <Unknowns> sometimes.