Is it normal for oxygen saturation to sharply drop when deep breathing/hyperventilation stops? Patient's SPO2 can drop into 80% region

YES. Hyperventilation raises PO-2 in normla lungs and afterwards the patient will UNDERBREATHE since the stimulation for breathing (Co2) has DROPPED due to the reapid breathing! To drop to 80% is completely normal (In my opinion) Hope this helps! Dr Z.