Check out this weird heart rate behavior

So lately when I run, I get a heart rate that steadily climbs on a perfect curve and tops out at 159 where I set my heart rate alert. My last few runs, the heart rate graph has been identical. It just climbs perfectly. I know my heart rate does not do this as I will walk and climb hills. My heart rate is usually all over the place.

Weirdly, when I'm not training it seems to be quite accurate. I have tried running it tighter and looser but neither seems to work. I've tried it on the interior of my wrist as well and that did nothing. Any ideas as to what is going on? Please look at the attached images to get an idea of what I'm talking about. Two separate runs