Why do my calendar events get deleted from my iphone while I'm viewing the calendar?

I was frustrated by this earlier in the year and assumed it was because of updates or updating devices. When I needed to check back on a date today I realised dates prior to 1 month had disappeared. I saw and followed the advice on the forum. See below:

Go to Settings...Mail, Contacts, Calenders

Go to Calendars section and scroll down to 'sync'

There you can set to 2 weeks, 1 month, 3 months, 6 months or All as preferred

i tried this and selected ' all' And all previous activities were retrieved from iCloud and were again visible.

thankyou so much.

I see this is an old post, but I have a similar problem when I add an event to my calendar on Macbook running 10.9.5 (I reverted to 10.9 when I installed 10.10 and it bricked my Macbook). It coincides with sync to cloud (which has been the only way to sync calendars on multiple devices for a few years). So I copy the event before it wipes it and then paste after it has wiped the event and the new one stays. This is much harder on an iPhone and has to be done one at a time. It is also not exactly the same problem but probably has the same root cause.

Go to Icloud in settings and turn on calendar, your events should reappear

Click on your calendar app, on the center bottom of screen click on "calendars" then click on "show all calendars" on the top center of screen.

Sorry... I meant ....even though my calender's settings are correct.