Erratic Heart Rate Drift Test

Hello, i’ve been working with an athlete for a few weeks now and i’m trying to get a handle on his heart rate zones. However, each test shows an erratic heart rate. It’s the third time he’s repeated the test and it’s a similar theme each time.

He reports back that he’s running relaxed and nasal breathing. He is also using a chest strap.

My initial thoughts were hardware problem, maybe needs a new strap, or is this something that is actually happening to his heart rate. He is going from highs of 190 to lows of 140 within 20seconds. Seems physiological not possible, but i’m unsure.

Love to hear if anyone has had experience this…