Do you have another computer in your home you can run Roon on? This will help determine where in the playback chain this issue is occurring. Have you reached out to Fiio support about this issue?
I believe the last firmware actually worked was 1.1.5. I was active in Fiio forum and regarding on this issue, it was never fixed in the subsequent newer firmware with no obvious reasons given. You also need to install the USB-DAC ASIO driver and select ‘Fiio X5 ASIO’ instead of WASAPI.
The same issue happens when you try to output DSD (DoP) via the coaxial output from Fiio X5III to my external DAC, it shows a DoP transport stream of 176.4/24 bit with no sound but some background hiss. Hope this helps.
It seems an up to date Fiio X5iii does not let you install a former firmware…
The downgrading process fails and the installation aborts every time I try.