I have had two odd things happen with the X20pro DAC syncing. One sounds like what you are experiencing. On occasion, I will turn on the system (the DAC stays on all the time), wake up the Mac Mini, and find no DAC connection...the sound comes from the Mini. Unplug and re-plug in the DAC (I did the mod that directly connects the transformers to the IEC so no switch), and all is well. The Gustard shows up as connected from the Audio Midi panel of the computer, and works.
The other thing...also on occasion, I go to start up the system, and find the Mini has crashed. I am not sure what is up, but I think the Gustard "locks" in some more intense way than my Tranquility did, both using USB connections. Lately, rather than putting the computer to sleep with Audrivana in pause mode, I click the Audirvana stop button, releasing iTunes. Not positive, but this may solve whatever has caused the crashes. I wonder if manually unlocking the DAC using the DAC setting panel might solve this also, but have not tried.
I have done a lot of mods on the DAC, and they continue, output board diodes remaining. Also still playing some with damping and WA cap and semi-conductor sticker pieces now and then, but it all seems really good. I am mostly running 256 DSD from 44/16 files.