No, not usually. You are right, it generally looks like a progressive dip. Anytime the monitor loses contact, the FHR should be confirmed against mother's pulse, to be sure. You said the resident listened. Like stood there and listened to the monitor or actually adjusted Mom's position and the EFM, then listened? IMO, before deciding on a stat CS, FHT should be confirmed, bt you're right, there's no way of knowing w/o checking maternal pulse.
Don't beat yourself up about it, just make it a point from now on to compare the two. Then you won't worry yourself unnecessarily.