I have tried to solve this problem myself but still not working.
It looks like this in Synchronization manager & Azure AD Connect Troubleshoot
Azure AD Connect Password Hash Sync does NOT sync to Cloud
I have migrated a few users with Azure AD Connect, but it does not seem like the onpremise password is synced to the cloud.
However, if i check Azure Active Directory the Account is synced from onpremise and there is no duplicates what so ever.