So, I tend to have to switch phones a lot. Not a big deal, but the major pain is the having to reset my Android Wear watch (for my case, it's my Moto 360 Sport).The back and forth ... I'm not sure if it's caused any issue, but I noticed with my last change to the Moto Z2 Play that the Google Fit app wasn't on my watch. I was having issues with the watch face keeping track of my Fit goals I'd set up. Was getting some 'timeout errors'. That's how I noticed, the app wasn't on the watch at all.
I tried resyncing apps. Nope, still not there.
Tried uninstalling Fit and then reinstalling it... still wouldn't show up.
Some googling on support forums showed that there's some glitch on later versions with Fit and Wear. (funny how it only showed up on this last switch over).
So, to fix, just go and uninstall Fit, then download an APK for a version that worked. For me it was 1.59. Check out Apkmirror.
From there, just be patient and you'll eventually see that it installs to the watch as well (took about 5 minutes).
Once it's installed you can update any update for the app and things are just fine.