Alerts disappear when an appointment is accepted in Calendar app

Ive had this problem a few weeks. I always make an alert for 30 minutes before an appointment in the Calender app iOS5 on my IPod touch. I also send an invite to someone to join me for that appointment. When they accept the appointment the alert changes to none. I have checked this several times. This means the alarm I have set doesn't sound and I miss the appointment. Please can someone check that this happens to them as well.