Random calendar events disappear from iphone, iCloud, and Outlook calendar

Hello frustratedwithios11 and welcome to Apple Support Communities.

I see you're having an issue with calendar events disappearing across devices.

You mention more than one person using this calendar and also it being used across various devices. You also mention that Outlook is involved.

Is there a Family Sharing account involved here? If there is, this may be something as simple as one person in the group deleting the event which would delete it across all devices using accounts tied to that family group.

Family Sharing - Apple

If Family Sharing is not involved, you may want to check out this support page from Microsoft about Outlook events disappearing. If the issue is the events disappearing from Outlook and the devices are synched, that would cause the events to disappear from all synced devices.

Disappearing appointments in Outlook 2016 - Microsoft Community

In particular, it appears that the Outlook setting to delete meeting requests appears to have an affect when using multiple synched devices.

Cheers.