I have a iPhone 11. My calendar has cleared itself three times within the last year. Idk about the first two time because I was never able to fix it. But this last time I was able to figure out how to fix it.
I had to go to settings-apps (you can search just calendar in the tool bar) - calendar-default calendar- then change to whatever option it was on previously. I had to switch around to figure out what one it was get one before.
Please check which is your Default email Account for the Calendar on your device? Settings > Calendar > Default Calendar
All the devices sync with an email client server which hosts your calendar. If you have a Calendar on iCloud that will not sync with Google Calendar. Settings > Calendar > Default Calendar [Choose what you want]. What you add (Event) on iPhone Calendar is synced with default calendar unless you have chosen a specific calendar at the time of event creation.
To which email account are the calendar events synced with your iPhone? This is not clear in your post
See the iPhone and Mac Calendar Pics below
Which is your default calendar? Settings > Calendar > Default Calendar
Settings > Calendar > Sync > [All Events] - Watch the GIF below
Calendar app keeps deleting my events
