I was able to reproduce this issue from GeoViewTapped Event Listener with specific points. The funny thing is if you run the same exact code where its not in an Event Listener/Button it will select the features just fine within the triangle.
Outside button/event listener:
Inside button/event listener:
I have also seen some sporadic behavior on this where if you click while zoomed in partly over this triangle it will again select correctly. You likely won't see that sporadic behavior when using the MapView's extent to create the triangle.
I will get started on logging a bug for this issue. If you let me know your customer number I can create a case and attach your organization to it so it will be trackable in My Esri.