SQL Server Distributed Availability Group Backup Configuration

We have a Distributed AG instance with two AGs, each with two replicas. The local AG Backup preference is 'Any' on each AG. Backup jobs run successfully on the global primary but fail on the Forwarder. Microsoft documentation suggests that it should be possible to run backup jobs on the Forwarder: > In a distributed availability group, backups can be performed on > secondary replicas in the same availability group as the active > primary replica, **or on the primary replica of any secondary > availability groups.** Backups cannot be performed on a secondary > replica in a secondary availability group because secondary replicas > only communicate with the primary replica in their own availability > group. Only replicas that communicate directly with the global primary > replica can perform backup operations. I have tried to research this issue unsuccessfully. I see other people have asked the question but there doesn't seem to have been a helpful response. Can anyone help please? Is the backup job expected to fail on the Forwarder? In which case, the backups take place **only** on the global primary? Also, should there be any concern about transition logs filling up on the secondary AG, from not getting backed up? Thank you.