The Heart rate displayed on my Apex Pro 2 just never makes sense.

It just never seems to be accurate at all. I can be at the gym lifting weights and look down at my watch after a set and it’ll say I have a heart rate of 40, which is ridiculous. I’ve been in the middle of a long walk and my heart rate will be at 35 one moment, 100 the next and then drop back down to 40 without changing my physical exertion at all. it makes no sense lol.