On iPhone X, iOS 14.3 the fix was to turn off motion & fitness tracking for Health. Health then showed the exact number of steps from Garmin Connect app on every sync. Settings>Privacy>Motion & Fitness…
I was able to fix this by going into Apple Health, Steps, Data Sources and then Edit and dragged Connect to the top right under Apple Health.
I disable the steps in the iphone by going into settings / Confidentiality / Movement & form / and uncheck follow ....
I had in the GC source setup of Health the ability to read data by GC. So I uncheck this to see if it is going to be better !
Change the priority order on the Health App. See this www.google.com/.../
Once you change the priority the Health app will have the same steps as your Health app.