Events in shared Outlook.com Calendars Now Sending Invites

We have an Outlook.com Premium account with a personal domain name that has now been incorporated into my Office 365 Family subscription. Both my wife and myself have shared our calendars associated with our Outlook.com emails addresses with each other. My events appear in her calendar and her events appear in mine.

This has worked well for a number of years. We have each been able to edit each others event for the purposes of adding notes or entering an address etc.

We both use iPhone 14's with the latest iOS 17.3.1 and the events show in iOS Calendar App

Recently however shared events entered by either of us are sending an invite so when we open an event it shows the recipient as an Invitee and Accept Maybe Decline at the bottom of the screen as per the screenshot below.

If you click any of these it says Your Response to the invitation Cannot be Sent".

I have gone into each of our Outlook.com web accounts and deleted the existing shared calendar and then re-shared them but it has made no difference.

I can edit or delete an event in Outlook.com for the web and also in Outlook 365 desktop app but not on the iPhone.

There are number of posts on Reddit with other users of iPhones who have updated their iOS version

Can anyone shed any light on why events have started sending invites as it is really frustrating.

NB we are both using iOS Mail & Calendar APPS NOT Outlook App for iOS

Many thanks