Try this (it worked for me).
Note: I did delete all instances of the calendar series before I did this. It is possible that is a necessary step.
- Open your Deleted Items folder in Outlook
- Find the original invite (if there were updates along the way, ignore those)
- Right-click the original item and select Move > Inbox (you may need to select Move > Other Folder > Inbox).
- When prompted to send this (for some reason it did not just move), email the appointment to yourself.
- When the appointment arrives, click the attachment and select Copy to My Calendar.
Hopefully this will solve the problem for you. In my case it returned all instances back to my calendar and I could delete the one I intended to delete.
EDIT: There is another method that works, even if you are in M365, and it is easier than my original solution:
- Open the entire series
- Click on Recurrence
- Extend the recurrence by an arbitrary amount of time (one week or one hour should do)
- Send updates (this resets all exceptions and restores deleted instances of the series)
- Go back and delete the correct instance(s)
- Go