If you check this out online with a swifty Google search you’ll find many reports and some solutions to a problem about syncing. For instance, this: Apple Support page.
But it’s not working for me. And given how reliant I am on this stuff, it’s becoming a big issue: calendar entries on my iPhone don’t make it to my iPad and vice versa.
I’ve no solution myself but wanted to warn you: check your calendar to see that it’s all copying across the way nature intended.