Had the same problems, and replaced the strap by a more breathable from Suunto. Still if I forget to loosen the watch after workouts and wear it all time I get skin irritation at the sensor area. Then I switch wrists at least during sleep.
I don’t think its the sensor but the charging poles maybe they contain some irritating metals and in combination with sweat and pressure it irritats the skin.
I haven’t been tested positiv on metal allergies but sometimes I have problems with metal buttons on trousers, sometimes not with the same button.