[Appointments+] Appointments going straight to Removed status

My client has Appointments+ installed. He has had several appointments that have gone straight to Removed status, instead of staying in Pending until Confirmed.

I have not been able to duplicate the issue despite repeated testing.

The client receives an email notification for each new appointment. He clicks on the link in the email, which opens the site with the list of Pending appointments. Only the new appointment is not under Pending, it's under Removed.

I'd like to know if a) anyone else has experienced this and b) do you have any suggestions on how to troubleshoot this, because I'm at a loss.

It's been going on for a few months.