The problem is you eliminated 2 distinct components, the DAC and the USB receiver.
Hard to tell "Who dunnit".
I'm inclined to suspect the USB receiver (adaptive mode probably) and/or the direct electrical connection between the PC and the DAC over the power and/or data lines of the USB.
Nice bit of guesswork anyway.
It would be interesting to try a modern USB implementation (asynchronous) preferably combined with galvanic isolation using the same source.
Would it be jitter either way?
P