Instinct 2 underestimating HR readings terribly

Is anyone else having problems with inaccurate heart rate reading when exercising?
Mine is underestimating by a huge margin and it’s been doing this for about 3 weeks.
I went for a cycle today and I know from experience that my heart rate would have peaked about 150 and exercise load would have been about 50.
It was accurate at the start of the ride and at the end but in the middle of the ride I was looking at the watch and my HR was 109-110 when I know it would have been high 140’s
This is really messing up my training load. Todays ride registered at 17 which would be what I woild expect after a 2 mile brisk walk.
It really pissing me off and I’m ready to throw it in the bin tbh.
Someone save me