I have the original Galaxy watch (42mm) that works perfectly with my S10+ phone.
I restored my Samsung Backup after updating my phone to an S21 Ultra, then re-paired my watch with the new phone (wiping & reloading the watch.)
Since then, the distance reported for a walk has been wrong by about 35%.
I documented today's walk as 1.99 miles using a second phone app. The watch and the Samsung Health phone app both indicate 1.3 miles, and reaching 1 mile takes 5 minutes longer than it does.
Also, the Health app used to log the route's GPS track, which I no longer see.
I adjusted the watch to utilise GPS solely for position and re-entered profile information that had been lost, such as my height and weight. It didn't make a difference.