Old calendar appointments have reappeared
My iPhone 8 syncs to my Outlook calendar at work. Recently, several old appointments (on some days as many as 5 or 6) have reappeared on my iPhone calendar. I only have the one calendar on the iPhone. They do not appear in Outlook so I am unable to delete them from there. I have tried unchecking the work calendar on the iPhone and then rechecking it in the Calendar section of settings without success. I'm running iOS 11.2.5 currently- previous iOS updates have not solved the problem. It's getting to the point where I can't see the real appointments because of all the noise from old appointments. Any ideas on how to solve this?
iPhone 8, iOS 11.2.5