Hi Kevin,
I am experiencing a similar problem since my latest upgrade of Volumio, just in a more severe form, as I believe it has to do with the USB traffic load. In my case the files are on a USB stick and the DAC is fed via USB. I have occasional stuttering starting with 16bit/48KHz wav files and the higher the bitrate the more frequent the stuttering.
In your case the USB bus is used only for fetching the files, you feed the DAC via SPDIF, hence your problem happens only at higher bitrates (DSD256 which is pretty high).
I was not having the problem on the same HW configuration before the update, running a version of Volumio dating back to May 2020.
Here is the post detailing my problem for cross-reference.