AD Connect Sync existing user's password are not synchronizing to the target DC
In case if the AD User accounts already exist on both Cloud and Client DCs then the passwords are not synchronized. What could be the common reason for it ?
Simple answer to this query is that existing AD Users passwords will not be pushed to the target Cloud DC because Microsoft has not exposed any API call or a way to retrieve the existing passwords. Therefore you have to reset the existing password to get the password syncing work for existing AD users.