We've had this issue for a client of ours numerous times.
They have a shared calendar that everyone in the business has access to, around 10 people. This is synced with Outlook desktop and iPhones using iOS Calendar. Everyone has full permission to read/write etc.
The issue they call with is as follows:
'Person A can't see all of the appointments for today on their iPhone but I can see them on Outlook desktop'
Removing and re-adding the account on the iPhone doesn't fix it, does not bring in the other appointments, creating appointments sync to all users, deleting those appointments also syncs across.
This is what I came across when trying to fix the issue. If I log into Office.com as the affected user and go to Outlook Calendar I can't see all appointments, I then create a test appointment, leave it a few and close the calendar browser...