For about the 3rd time in a month, my messages coming into my Series 4 do not have the contact, just the number. I tried the re-sync option and it didn't work. Had to unpair and re pair. Irritating. Bug?
What watchOS and iOS are you running? I had this issue but it was eventually fixed after a few releases. Update to the latest iOS and watchOS if you haven’t.
Watch app>General>Reset>Reset Sync Data
Unpair and re-pair the device
Watch app>General>Reset>Erase Apple Watch Content and Settings