If I enter a calendar event on June 7, 2016 or earlier it works as expected. If I enter an event on June 8, 2016 or later it briefly appears on the phone then disappears. The event still exists when I check Google calendar on a PC.
I noticed this yesterday and checked this morning to see if the cut-off date moved forward one day but it didn't. I'm in the US in the Eastern Time Zone. The phone is a Lumia 640 and the calendar app version is 1.0.15087.0. My wife has the same phone and sees the same behavior.