Why is my Apple Watch S9 cycle track not working?

I’ve had my watch (S9) for 6 months.

Wrist temperature has tracked with no issues continuously all that time, until 1 week ago.

Position - exactly the same.

charging - always 50% or more.

sleep tracking - set up fine.

I’ve reset the watch, and turned the temperature tracking on and off, and reset as well.

Nothing has worked. Any ideas ?

[Re-Titled by Moderator]