I started having this issue, after some time I bought the SG Apple Watch Series 2.
I talked to Apple Care regarding this issue, they suggested that I Unpair Apple Watch
from my iPhone, then Re-Pair my Apple Watch with my iPhone. Then, setup as a New Watch,
and NOT restoring from old backup. It worked like magic. That was the only solution that worked
for me. But the problem is, this solution is not permanent. After a week or so, it occurs again.
Today, I started using navigation to an address on my iPhone, after I started Maps app on Apple
Watch, it started guiding me step by step to the final destination with Maps, but in the middle of navigation,
Apple Watch starts again showing grid lines instead of actual maps for half an hour. After that
it started showing maps again instead of grid lines. I wonder, if I setup my iPhone as new iPhone,
instead of restoring from recent backup, Could fix this issue ?