Also, I believe installing firmware-intel-sound changes the channel mapping, before it speaker-test -c 6 works. After it the mappings are wrong.
I have a 6.4.2 setup, speaker-test -c 8 fails with
Cannot open WAV file /usr/share/sounds/alsa/Channel_13.wav
Does it help to reset mixer settings and re-init the device:
sudo rm /var/lib/alsa/asound.state
sudo alsactl init
root@DietPi:~# sudo rm /var/lib/alsa/asound.state
sudo alsactl init
rm: cannot remove '/var/lib/alsa/asound.state': No such file or directory
alsa-lib parser.c:2783:(load_toplevel_config) Unable to find the top-level configuration file '/usr/share/alsa/ucm2/ucm.conf'.
alsa-lib main.c:1541:(snd_use_case_mgr_open) error: failed to import hw:0 use case configuration -2
Found hardware: "HDA-Intel" "Intel Tigerlake HDMI" "HDA:80862812,80860101,00100000" "0x8086" "0x3002"
Hardware is initialized using a generic method
That got the channel mapping for -c 6 back correct, except I didn’t hear the noise from the subs, sounded like a voice.
Okay, the /var/lib/alsa/asound.state is created when using alsamixer or amixer, or when doing alsactl store. dietpi-config does the latter when selecting a sound card, actually 
. However, it should not be relevant for Roon, which does the mixer stuff internally, if I am not mistaken.
UCM is the ALSA Use Case Manager (alsa-ucm-conf package), which is not installed OOTB with ALSA. It contains configurations for specific audio devices, though nothing matching for your case, AFAIS: Debian -- Details of package alsa-ucm-conf in bookworm
On Trixie the list has become longer already, but still nothing matching (?): Debian -- File list of package alsa-ucm-conf/trixie/all
The alsaucm command can be used to select such configurations. alsactl init weirdly shows this error, if UCM is not installed, which can be avoided by explicitly skipping the UCM config init with alsactl -U init. However, just want to say both errors are not related to the issue.
So we know after installing new firmware or a new kernel, alsactl init should be done reset all audio device states. Does speaker-test -c 8 work as well again?
root@DietPi:~# alsactl -U init
Found hardware: "HDA-Intel" "Intel Tigerlake HDMI" "HDA:80862812,80860101,00100000" "0x8086" "0x3002"
Hardware is initialized using a generic method
speaker-test -c 6 works as expected, speaker-test -c 8 still has problems with the channel mappings.
Since I was not able to get this issue resolved, I’ve gone to a Windows based solution. It is able to play multichannel flac, but multichannel dsf still fails. Failed to initialize Audio device with multichannel DSD (ref#EVAGFB) - #12 by David_Phillips
You can’t play DSD over HDMI from a PC it needs to be converted to PCM by Roon is it doing this?
Yes, dsf is converted to pcm, but multichannel dsf fails when the Roon bridge is set to multichannel. If set to 2.0 everything works fine. The other thread has more information and pictures.