iPhone Calendar Picking Up Events Deleted Years Ago

We had this issue months ago where the iPhone calendar refused to delete long ago deleted Outlook calendar events.

In that instance, the events had been valid, but simply did not disappear from the phone when deleted from Outlook.

I fixed the issue, and those events have been gone from all calendars for *literally* months.

Last night, the events reappeared on the iPhone calendar. These are events that were recurring meetings that were deleted from Outlook *literally* years ago.

The only possible explanation I can come up with is that the iPhone has a cached version of the calendar that it somehow reverted to due to an update or something.

To be clear, these events don't show on ANY other calendar and have not for years. (Not Outlook in Windows, now OWA, not Teams, nowhere except the iPhone).

These events were gone from the iPhone calendar until last night.

The events now show even in the past, where they did not show before last night.

Deleting events is not successful, individually or the series. They do not delete from any day except the day selected, and even then if I leave that day and return, they are back.

Anyone know where this "phantom" calendar is stored and if there is a way to kill it permanently?