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.
Try uninstalling & reinstalling the app. That often fixes that kind of problem.
This is not happening in my configuration ....
The internal sensor of the iphone is deactivated, yesterday I made only 3958 steps in Garmin connect , I verify this morning, in the Apple Health 1969 steps ....