I have the exact same issue. I brought the watch into a Samsung store and they said they have several people bring their watches in with this exact same issues. Since my watch is now past the warranty coverage I cant get it fixed for free. They quoted me approximatelt $100 to put on a new plate but if there is any kind of moisture that got in, it is not fixable. I have contacted the support on Samsung and they said, "I’m sorry to hear that the protecting plate on the watch fell off. However, there are no such reported issues or recall with this model." Which is ridiculous because there have been several posts about sensor covers falling off on gear sport model and the galaxy model watches ( https://us.community.samsung.com/t5/Galaxy-Watch/Galaxy-Watch-Sensor-Cover-Fell-Off/td-p/649307/page... )
This is definitely a design flaw as the new active watches have redesigned the watch to not have a sensor plate held down by adhesives which I think isridiculous for a "waterproof" watch. Adhesives wear down after time so why would they choose to use this to encase a waterproof "sport" watch that should last through the harshest of conditions.
I would be okay with Samsung even sending me a replacement sensor plate so I can at least crazy glue it to the back of my watch so the heart rate sensor can work but they refused. I now just taped up the back so I can wear it without the sensor adhesive touching my skin.
Very disappointed in this product. I have many Samsung products that are great but the fact that Samsung wont take responsibility for their design flaw is very upsetting. Would not be buying anymore Samsung products in the future. Their support is terrible.