I was just writing a post, but I think @RBM may have nailed it. I believe the indicators on the DAC are showing you the sample rate of the input, not the upsampled output.
I would want to see a screenshot of your output settings to confirm, but assuming you have things set up properly and are seeing the purple “bit perfect” light in Signal Path, Roon is delivering a bit-perfect stream to the DAC, and the upsampling happens in the DAC.
If your system output is set to upsample then the indicators would match what’s described here, meaning upsampling is actually happening in the Mac Mini, in addition to any upsampling happening in the DAC.
Take a look at the Mac Mini’s output settings, and let us know if it sounds like we’re on the right track here, @David_Salsberg. Thanks!