The up-sampling is a DAC feature. As you don’t use the DAC it seems not to be relevant for your use case. It looks like the digital input signal gets just passed trough to the digital outputs of the 851N. There is usually no conversion done (unless for compatibility reasons?) much as Roon handles it on default settings. I guess that the 851N does not signal the output in use (it most-likely doesn’t know that himself) as the outputs often work in parallel and the user is free to uses which ever output he wants.
So while the up-sampling seems to be true for the analog outputs (prior to the D/A conversion) it seems to be not true for the use of the digital outputs. A signaling of the output in use is not happening (and maybe impossible). The signal path in Roon seems to be wrong here, showing that the up-sampling is taking place before the “Output” through “Analog/Digital Outputs”. Have you clicked on the blue star behind the “Sample Rate Conversion” entry (this might reveal additional information)?
As all this is discussing internals of the 851N, have you contacted Cambridge Audio also?