Tracking Steps in Apple Health

For the past several years I've used my Apple iWatch 3 to track my steps, exercise, stand goals and transfer it from my watch to Apple Health, then out to other apps like myFitness Pal. They always matched up pretty accurately.

Last week I got a new iWatch 6 (LOVE IT), but now my steps aren't accurately syncing. My watch shows today that I took 4,236 steps after a run on the treadmill, my Fitness app on the phone shows the same 4,236 steps, but the Apple Health app only shows 2,849 steps. It's had similar differences each day since I transitioned watches, and I can't figure out why.

And based on other threads, I've already checked that the new iWatch is the #1 source in Data Source & Access that my new watch is the #1 option for steps. What might I be missing?