I'd been having the same problem with Samsung health not always syncing my step count to My Fitness Pal and after reading all the comments and trying a few people's suggestions it was still not working. I then started having a play around in the S Health app and done the follow which worked!... Not sure if it was a coincidence or actually worked!? Will see again tomorrow.
Scroll to bottom of S health home screen > click 'Manage items' > scroll down to 'feature apps' at bottom of page > click find more apps > it should then show My Fitness Pal (mine says 'open ' in green) > I clicked on it and it opened up MFP and updated the steps!...
Goodluck fingers crossed it might help some of you! 🤞☺️
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