Sorry you're having trouble with forwarding. I'm not able to offer any advice on troubleshooting this; however, I do have a suggestion for a workaround while this get replicated and fixed.
You can use content filtering to copy or reroute a message to another mailbox. This will give you the similar functionality found on the Forwarding tab. Go to Settings | My Settings | Filtering | Content Filtering. For the conditions listed on the first page, choose "My address in to or cc field." Skip past the next page, and for the actions listed choose either to "Copy message" or "Reroute message". If, when using the Forwarding tab, you chose to "Delete messages when forwarded" you'll want to choose Reroute Message as your action. If you want a copy in both accounts, choose Copy Message.
Now, one limitation of using content filtering is that you can only enter one email address in the Copy or Reroute field. I tried it with two and the message still arrives in my SmarterMail account but is undeliverable to the forwarded accounts. That being said, if you're forwarding to multiple addresses you'll have to create a separate rule for each. I understand that can be a hassle for those who have a large number of email addresses they're forwarding to, but hopefully for most of you it can be a good workaround until this is resolved.
Hope this helps!