I have noticed that in the last 1 or 2 months, sometimes one event here and there, disappear from the calendar. Luckily I have good memory to remember some of them, but not all, and cannot rely too much anyway! I remember one event in particular, it disappeared, I put it back. I checked my mobile after few hours, it was gone again, I put it back and it stayed this time.
I really don't know what is the cause. I think that the issue doesn't occur on repetitive events, while it does for the once off ones.