Watch HR not accurate during treadmill stress test?

I have had two treadmill stress tests recently and my Apple Watch series 3 did not pick up my high heart rates either time. Both times I was hooked up to a 10 lead EKG and my heart rate rose into the 170's. On both occasions though, my watch only picked up my HR as being in the 120's. Why would this be the case? This is concerning as it makes me wonder about the accuracy the rest of the time...