It’s not my heart rate measurement that has the issue. The device has an accelerometer in it to that measures running movement metrics, so it can determine things like stride length, vertical movement with each stride, ground contact time, and the ratio of time spent on each foot. What I think is happening is that because of the positioning of the strap, just below the breast crease, incidental breast movement is moving the strap and sensor enough to impact the accelerometers measurements.
Can’t hurt to try I guess! I’ll give it a go