I have the same problem with Apple Watch S4. Earth complication does not update.
Can only force an update by going into the Customise panel and then quitting out, or by swiping to another watch face and then back.
Watch is on WatchOS 5.1.1, and iPhone is on iOS 12.1. These are the latest OS versions available at the time of writing this post. The problem has been there when I purchased the Watch in mid-September, 2018, and it still exists after two WatchOS updates (5.0=>5.0.1=>5.1.1)...
Tried rebooting iPhone and Watch a few times, which didn't help.