why do I need to enter alert several times?

Just wondering why - I believe this has been since iOS 9.x - most of the times, when I try to change the default ''30 minutes before'' when entering an element on caendar (iPhone 5S) for another time, and especially if I try to set 2 alerts, the default come back instead of the alerts I have set and I need to do it 3-4 times before it stays ! It is very annoying...

For some times, I suspected it was only when setting elements to be shared in family calendar, but it is happening also on most of things I only set for my own calendar.

Exemple:

I set an element in calendar, want an alert say, one week before, than another one one day (or 2 hours) before, it will get back to 30 minutes (often, both alerts..) I need to re-enter what I want 2-3-4 times before it keeps it.

Am I alone to have this very annoying bug ?