If a customer is using a VMware-based Veeam backup proxy and attempts to perform a direct backup from their Fibre Channel (FC)-connected HPE Alletra 9000 array, but the backup falls back to network mode, the likely reason is that direct backup from an FC-connected array is only supported on a physical server-based backup proxy. VMware-based (virtual) proxies do not support direct FC connections, which is why the backup process defaults to network mode.
To achieve direct backup via FC, the customer would need to configure a physical server as the backup proxy.
For further details, refer to:
HPE Alletra 9000 Overview
Veeam Backup & Replication Documentation