Hello.
This issue was already discussed 4 years ago in this forum but is unfortunately still not solved!
I was working on the challenge of achieving my daily steps during 30 days and had 27 days in a row. However yesterday I got up to my daily steps and went to bed and today my Garmin Fenix 6 and the connect app showed about 1300 steps less and I missed the challenge because of this bug. (luckily I have screenshots of this as I realized earlier that the app was sometime not properly syncing) This is so frustrating and unfortunately Garmin has not been able to solve this random issue within 4 years!
As you can see in my 2 screenshots I achieved my daily steps on March 18. but in the 4 week overview and today it says on my watch and the app that I did not achieve my daily steps which is not correct!
Has anyone else had this issue before? I will contact the support tomorrow.