The CR for this 794128 and the bug has now been identified and fixed for 15.7 SP137 (and I assume some release of 16).
In essence the number of wait events for 215 (only occasionally pulls back 1 only) and the lack of persistence of the row in monProcessWaits means you can't really track this event using that table.
Post-fix, once SP137 is released, the capture of wait event 215 will behave comparably to the closely related wait event 214 (which works as expected on the current release).