Low heart rate on climb
I have seen this effect with several different optical based wrist HRM's. Your HR is not 90, but likely closer to 180. Optical HRM's generally have a longer lag to track changes in the heart rate and sudden jumps can be interpreted by the software as drops rather than increases. 

I've found that for long sustained efforts, the HRM eventually tracks back to the correct value, but can be wildly off for shorter efforts. There is also the physiological lag in HR vs effort. Early on in a workout, your HR doesn't track your effort particularly well.  

Many of the coaching/training blogs I follow are suggesting that perceived effort should always trump HR based training, particularly as HRM's struggle to track correctly at high heart rates. If you're breathing heavy and think you're working hard, you're working hard regardless of what the HRM says.

For a recreational athlete I'd suggest just living with it. I find the convenience of wrist based optical sensors far outweighs the occasional data inaccuracy. Most professional bike riders still use electrical chest based straps for this reason, but they also all use power meters to gauge their efforts during training. 

If you want to learn more about the limitations of your device, I recommend searching for reviews on DCRainmaker.