I have the same issue. I have one cam pan running latest firmware 18.104.22.168 and a iPhone and iwatch. Random alerts appear on watch. Maybe 5%. Seemed to start when I eventually updated the camera firmware to 22.214.171.124. Not sure how that could affect the alerts showing on the watch. Checked the app settings on phone all is good.
For a short period yesterday, I got motion alerts on my Apple watch! \o/
Today, not so much.
Last year, alerts to the watch worked fine.
Around January, it stopped working (alerts on phone only).
For a few hours yesterday, it worked again.
Today, no alerts on my watch
I performed no config changes or software updates on any device this week.
This implies to me that the error is on the cloud side. Malformed push notification?