In two different environments I have reproduced behavior where Azure AD Connect does not synchronize passwords when it is configured for multiple source AD forests.
The fix has been to change the ‘Configure Directory Partitions’ credential setting from ‘Use default forest credentials’ to ‘Alternate credentials for this directory partition’
No service restart or reboot required. The way to test it is to reset a password and then monitor the Application event log on the Azure AD Connect Server. Within 2 to 3 minutes you should see an event log entry that the password has been successfully set.