Welcome Mark, fellow user here and likely not able to help at all, but please allow some questions out of curiosity anyway.
How many channels do you actually use?
Do you do any channel mapping in Roon?
What happens, if you set up your Grace to 8 channels for all sample rates?
Maybe it could help to show some screen shots of all device setup info in Roon.
My guess is that there’s not much Roon could do to help with the issue, since it’s not a Roon Ready device connected via network (Grace obviously did not apply for certification), but only seen as a USB-DAC.
Good luck anyways, and I’d really like to see pictures of your setup in this thread.