My DAC is a Mola Mola Makua. The testing I reported previously was done on that but here are more clues...
I had a Klein DAC on loan early today. The owner says it works with native 4xDSD via JRiver on Windows server. Like mine, it can do 4X DSD only via native DSD so that is a test for true native DSD. I could not play 4xDSD from my Snakeoil server to that DAC either (may have only tried dsd-rt; can't remember). It never worked on his Snakeoil server either.
With Roon, it was my understanding that the option to select "native DSD" would appear if Roon detects that the DAC can handle native DSD. No such option presents for either the Klein DAC or my DAC when using Roon on Snakeoil.
Maybe it is a Snakeoil issue or maybe some DACs don't advertise their native DSD abilities to any server. A way to force native DSD would be great.