I have spent hours trying to figure this out. For years I had outlook 2007, I would plug in my iPhone via USB, click sync in iTunes, and my calendar would sync both ways. Worked great.
I just got a new computer and with it Office 2016. I use Outlook but my company has an imap server, I do not use outlook.com. When I try and sync like before, it only puts appointments from my current computer onto the phone, not vise versa. I did make sure I was adding appointments in the correct calendar on the phone. I tried changing old appointments to the new calendar and adding new ones.
I tried using iCloud for windows (never used it before) and all the appointments showed up for n the computer but I can't make iCloud my default calendar. I get about 5-6 calendar invites a day and I have to manually move them to the cloud calendar. This is not a good solution.
Before I retired and was still working in IT, I ran into this issue with some clients in that the calendar would not sync any more with Office 2013 and at that time it was an issue with iTunes not working with newer versions of Office/Outlook for that function.
By the time I retired only one client was still using iTunes to sync his stuff with Outlook and everyone else was using Office 365 or other hosted Exchange email.
You might try contacting Apple to see if anything has changed but I would not be too hopeful that it has.