@markeczzz interesting observation.
Do you have an idea what could be different for the watch when you are climbing?
The watch is basically trying detect the capillary blood flow.
Bad measurements could come from a too tight strap (blood flow interrupted), a to loose strap (watch shaking and thus variation in the optical path), cold temperatures (capillaries have less blood), using poles (introducing rythmic vibrations of the watch), tatoos, etc.

What parameter (within arm/watch) are changing when you climb? Maybe you can observe anything?