Why aren't calendar events being read on my Samsung Galaxy S6?

[AC Question]

Calendar events are not being read on Samsung Galaxy S6

My mobile app is able to access and read the calendar events on Samsung galaxy S5 (on version 5.0 and above) but not on the Samsung galaxy S6.

The necessary permissions are provided (and hence the app works on S5)
android.permission.READ_CALENDAR

The Galaxy S6 has the default Android calendar version 3.1.4.*** thought whereas the S5 shows calendar version 1.0 .

The calendar synchs and received events correctly to and from Google calendar though.