UPDATE: Talking to Rose in Customer Service, we found there is a Control panel item for the Domain that determines which emails can Forward. 

Much like the newly introduced issue where spam thresholds reverted on update, this area may have also changed (reverted to default) which is what confused our understanding of the problem. We were seeing some emails forward and other not, as a new condition.

Making the change to this setting has returned Forwarding to the expected case.

Good Customer Service from Rose!