Play DSD over PCM through ipad pro 2018 (USB type-c interface) will provide large noise with fading music on the background. No problem if I replace the ipad pro with any other iphone and older iPad.
You have no need to replicate my whole system. Just try to connect any USB DAC to ipad pro 2018 USB type-c interface, probably the problem can be replicated.
My setup is
[Music library+Roon Core@synology NAS ds918] —(wifi)->
[Roon Bridge@ipad pro] --(usb-c) ->
[USB TYPE C hub with power delivery (with mechanical timer on at night for charging)] —(usb-a) -->
[Audio-gd NFB-11] -->
[Headphone]