I have the same 2012 mini and notice the same issue with DSD256, sometimes constant dropouts with HQP and no issues at all with Roon.
The other issue I get is when I use NativeDSD with HQP I get a loud pop at the beginning of each song (not always, but most) and with Roon/SOSE the loud pop seems to only happen on the initial play when I turn on my DAC…apparently there is a fix for this, in the upcoming Sonicorbiter 2.5 software update.