Same issue here. It all stopped to work between our family members with iOS devices and Outlook Office 365 client. On August 13th, around noon, the calendar (and group feature) simply disappeared from everywhere (except if we login to hotmail.com and go
into the Calender, the group "Family Calendar" is there.
I've opened a ticket to Microsoft's customer support and the technician finally it seems like a server issue (since I've mentionned that I've tried everything, including removing / re-addin the Outlook addresses into the iOS devices and/or Outlook clients).
Creating a new Outlook profile didn't help either. Upgrading to latest iOS version didn't help.
The technician didn't have an ETA for a solution for us.
What I found weird, the technician wrote that the information is not available to the public.
Is it because we are using the free version of the Exchange service?
Using a hosted Exchange Server for many years prior to using only the free hotmail's services, I never had those kind of issues sharing calendars.