The Network Bridge has no way of knowing the downstream capabilities of the DAC so all that it can report is that it is performing the decoding stage which Roon tags as “Lossless”. With the Upsampler in use (or with your Rossini) the network processor knows what’s happening downstream and reports it per Roon’s requirements.
This behavior started with network firmware 398 which was released last fall. It carried through to 406 (released in January) and will continue unless Roon changes their requirements again.
There’s more information in the 398 release notes (download the PDF at the bottom of this post):