Using graph API to create calendar entries that are "Teams Meetings" in a shared calendar account, used to work but isn't now

I am using the graph API to create Calendar entries in a shared Outlook calendar account.

These are working successfully. I have been successfully creating them as Teams Meetings - I have been seeing that when I create the entries and say:

"isOnlineMeeting":true,
"onlineMeetingProvider":"teamsForBusiness"

Then the entry is successfully created as the Teams meeting in the shared account and shared accordingly amongst the attendees of the event.

This has been working successfully as noted.

However, with no changes in our configuration or in the JSON that we are passing to the create event call, we are now finding that the events are being created successfully yes, but they are not marked as a Teams meeting. We receive the following back from the POST call (which has passed the settings as noted above):

"onlineMeeting":null

So it is not successfully creating what we asked it to do and yet it's returning a successful response. I can see in the Outlook application that the events have been created but they are not marked as a Teams meeting.

We can prove in our audit records that this has been working successfully in the past but is no longer working despite no modifications.

I have checked the format of our JSON carefully and the security of the accounts and we have not changed.

I have read several identical type questions in this forum where people have noted the same issue and a few have noted that it seems to be an intermittent problem.