Heart rate inaccuracy during treadmill

Hello community,

I've noticed lately (at least the last 3 times) that when I start my treadmill activity, the heart rate remains around 70-90 and then all of a sudden after 15-20 min, my heart rate goes to 140-160. I know the accuracy during the first minutes is not accurate and I wanted to know how I could solve this or does the watch have a defect?

I wear the watch and maintain it according to the standard instructions.

Thank you