I have no idea if my latest experience with my iPhone and Garmin Connect echoes what you're experiencing. I'm trying to find the correct forum to share this...
"Merry Christmas:) After doing the latest IOS 26.2 update, I've being having little luck syncing my health date, steps, sleep, etc from my Venu 2s to the Garmin Connect app on my iPhone 13. The watch shows my heartbeat in real time, but I can only access the information on my watch and not in Garmin Connect. I've tried all the old fixes of turning on/off both devices...factory resets, etc. I had a long sessions with Apple Support who walked me through all the possible problem areas on the iPhone but everything is working at the Apple end. I then chatted with Chat GPT but ended with "Garmin Connect does not appear under Motion & Fitness, so it cannot request permission and no data syncs. Reinstalling the app does not trigger the permissions prompt. This appears to be a Motion & Fitness entitlement / privacy database issue in IOS". The techs at Apple disagreed. So, I'm stuck. An help would be appreciated. Cheers! Kev"
Good luck finding a solution to the OP's problem. Sincerely, Kevin