Prednisone has far reaching effects on your heart.
https://www.verywellhealth.com/prednisone-heart-rate-5113682#:~:text=Prednisone%20is%20a%20steroid%20drug,irregular%20heartbeats)%2C%20and%20palpitations.
I don't wish to alarm people but don't let anyone tell you that prednisone side effects are easy to manage if you know how. The problem with cardiac side effects from prednisone is they are usually "silent." You don't really know what your heart is doing most of the time.

People sometimes feel a racing heart but that can be attributed to exercise, stress, or simply getting excited about something. It is good that your smart watch is monitoring your heart rate for you. Calculating your average daily heart rate is good because your heart rate changes all the time.

My "normal" resting heart rate was below 60 but I never had a smart watch back then. By most standards ... below 60 would be called bradycardia. On prednisone, my resting heart rate was in the 70's which is considered normal. Nothing to worry about except a resting heart rate in the 70's wasn't normal for me.

I won't go into all my cardiac problems that I developed while on Prednisone. I didn't have many symptoms because I was mostly "symptom free." My very first symptom was "exercise intolerance" but that is subjective. I complained about not tolerating exercise but tests said I had a "normal exercise tolerance."

The culmination was when I noticed my pulse was irregular along with being short of breath ... not to mention being absolutely exhausted. This all happened while doing something that shouldn't have been so exhausting. I told my primary care doctor who wanted to do an EKG.

The EKG detected left ventricular hypertrophy (LVH) which is usually caused by high blood pressure except my blood pressure. I never felt my blood pressure and it was always normal whenever it was checked. Similar to catching a cardiac arrhythmia, which people also don't feel most of the time, I was instructed to check my blood pressure more often.

My blood pressure was "normal" until I caught a blood pressure reading that exceeded 210/110 for no apparent reason. That got my attention and my blood pressure was verified in the emergency room. I didn't feel anything but I soon had a cardiology consult. The cardiologist started three blood pressure medications. The cardiologist said my heart was being "stretched" and was getting "irritable." I had no idea that was happening.

My problem was the erratic nature of my heart rate and blood pressure when I was on prednisone. I never felt anything. It was like my body had lost control of how to regulate those things.

I'm off Prednisone now. My blood pressure medications were stopped one by one. Things aren't so erratic anymore.