A cardio workout increases heart rate to a safe level in order to strengthen the heart. It is not clear how a heart rate could decrease during that time.
Your heart rate is the number of times your heart beats per min. Not knowing your age and medical history. This is something you might want to discuss with your doctor. I hope this helps.
This may be an early sign of a heart condition or heart disease. My advice would be to check with a certified General Practicianer Medical Physician to check and monitor your hearts functionality and health.
This depends on how much your heart rate is dropping. If your heart rate drops significantly as you increase the intensity of your workout, consult your doctor for testing.