If the body sensor is not detected, HR measurement is not working.
This is a problem in a watch device that do not support the off-body sensor and this should be improved in a second update sometime this Autumn.
It is recommended when you publish your watch face application exclude the devices that do not support off-body sensor. You can rebuild and update after the next WFS release.
We are aware that the OPPO Watch does not support the off-body sensor there may be others.
Samsung Developer Relations