Garmin Connect app not sharing info with Apple Health app

Garmin doesn't really integrate with Apple Health. If Garmin says it does, then the most charitable way to characterize that statement would be an "exaggeration". In my experience, Garmin will occasionally sync activities and steps, but never reads anything (like weight). Sometimes it will sync calories from a workout, but not the workout itself. A more reliable path is to sync Garmin to Strava or another app, then open that app on the phone which stands a better chance to writing data.

I honestly think that Garmin had to work harder to come up with this situation than simply syncing with Apple Health. There's no reason why a sync should work on Monday but not Tuesday with no software or OS upgrade in between. So its just really hard to imagine what random bits get set in their code to make it work sometimes and not others.

But after a week of frustration, I realized that I don't really care.

TLDR: if syncing with Apple Health is a must-have, then Garmin probably isn't for you.