I had the phone away from my watch again for about a day and once again, it lost the info for the sunrise/sunset times when it reconnected back to my phone.
Tried the previous suggestion of turning off bluetooth while connected to wifi and then turning on bluetooth but it simply did not work this time.
Clearly some weird bug on the phone or watch software preventing the sunrise/sunset times to refresh because everything else e.g. location for weather, location on maps app seem to be fine.
Tonight reread this whole thread as I was getting very frustrated and desperate because some techniques which worked for myself and others previously, didn't seem to work again for me. e.g. A few days ago, I tried James Bucanek's method of launching the Maps app on the watch, and straight away, it got my location and sunset/sunrise times came back on the watch faces, but then, when that info disappeared again a few days later, the same method didn't work.
Finally I tried a combo of some of the suggestions already mentioned on this thread and documented all the steps. And... the watch faces for sunrise/sunset showed up, as did my location on the astronomy face. I can't guarantee it is repeatable but it worked tonight anyway.
1. On your iPhone, Under Settings -->Privacy --> Location services -->Apple Watch Faces, switch allow location access to "never"
2. On your iPhone, Under Settings --> wifi --> Forget your wifi network and then rejoin the network.
3. On your iPhone, Under settings --> Bluetooth, turn bluetooth off.
4. On the Watch, from the apps collection, tap on the Weather app. (When I did it, it showed the disconnected from iPhone symbol). I left the watch on this screen. Meanwhile...
5. On the iPhone, turn bluetooth back on. (When I did this, the weather app refreshed and loaded the weather for my current location.)
6. On the iPhone, go back to Privacy --> Location services --> Apple Watch faces, and switch allow location access to "while using the app"
When I hit the back arrow to return to location services main page showing the list of apps, the listing for Apple Watch faces app had a purple icon next to it indicating it recently used my location. This was promising.
7. Finally, on the Watch, go to your collection of apps icon, find and launch the Watch app which will bring up your watch face. Hopefully you should have your sunrise/sunset back, as well as your correct location on the Earth/astronomy watch face, and the times for dawn, twilight etc on the sun tracking watch face.
Good luck! Hope it works for you if you're having problems with this issue.