Deleted Google Calendar event not being deleted from external Office365 calendars

[phyxiis]

We have a person who is running into the following issue with Calendar.

They create an event and share it with external users at other Universities (it's about 50 or so people). Some use Google Workspace, some use Office365.

When our user deletes the event, the deletion reflects on external Google Workspace users, but not on the external Office365 users calendars. When the Office365 user goes to delete it from their Office365 calendar, it looks as though it deletes it, but then it readds it to the calendar so they're unable to remove the event from their Office365 calendar (witnessed during a Zoom call today).

After talking with Google support, they're saying most likely there was an interop set up on the Office365 side (there's no interop configured in our Google environment).

I find that hard to believe, considering if it's 30 local Universities, there's no one at our location who would have requested the external Univ. to set up some interop for calendar invites to be synced when deleted.. which is what Google support was saying.. I'm not confident in Google's response. 

Has anyone ran across this issue or have any insight as to what may be happening?

I believe at some point in the recent past this worked (deleted google event also deleted from office365 calendars) as intended, but at this point our user is having issues with it (this has been their 3rd notice to us of the issue happening).