Yup – when grouping, RAAT will fall back to the lowest common denominator, I guess:
As you can see, you can make things a little better by upgrading your Pi to the NEXT (development) kernel (currently 4.4.6) – this will give you up to 192Khz on the on-board audio (mini jack & HDMI), but the signal will still be truncated to 16-bit.