Apple notified the official and some third-party retailers about the problem with GPS in some Apple Watch 3. It is present only in the version of the watch without LTE (apparently due to the different antenna soldering scheme) and becomes noticeable with the display off.
You can see short stripes on the screen if you rub it with your finger when the device is turned off. Apple also advised retailers to ask shoppers to breathe on the edge of the display to create haze on the surface and check again. If a problem is discovered during use, then such a case is recognized as a warranty service and the watch will be exchanged for a new one. Recall that the official warranty period for hardware components is 1 year.