Apple Watch Series 11 SpO2 measurements suddenly fail everytime

Apple Watch Series 11 SpO2 measurements suddenly fail immediately after starting manual measurement. Last successful measurement was taken on 31/10/2025.

Steps taken:

-restarted both iPhone and Watch

-cleaned back of Watch.

-switched Watch to opposite hand

None of the steps above worked.

Measurements work fine with older Series 8 Watch without problems. Switching watch band from older Watch to newer did not correct issue. The watch has been in use one month. Running iOS 26.01 and watchOS 26.02