Cancellation emails need to send automatically
The email settings under "Event Registration Canceled" says "Sent when registration is canceled by the registrant, by an admin, or automatically" but the email does not send to the registrant when their registration is cancelled by the admin. I find this misleading as it looks like an email should be sent as soon as it is cancelled regardless, not by having the admin need to click send a notification. Also there is no prompt indicating that the admin needs to do so when cancelling.
Coupon Donor commented
Very helpful post