Snapcast (client) and Stereo upmixing to 5.1

I'm trying to configure upmixing stereo audio stream received by snapclient into 5.1 audo stream using Alsa. For that purpose I've created corresponding pcm device in asound conf. (Actually the same asound.conf I use last several years without any issues on ArchLinux, but on LibreELEC it doesn't work)

I've also confirmed that upmixing pcm device works with "speaker-test -D upmix_20to51 -c 2" command.

But when I start snapclient and specify "upmix_20to51" as sound card (snapclient -s upmix_20to51) the sound doesn't go through this device and go through the pcm.default one.

In the snapcast log I can see the following error: "soundcard "upmix_20to51" not found"

Any suggestions what can be wrong?

Also found this thread Snapcast & Hifiberry Digi issue where @awiouy said he was able to configure snapclient to use custom pcm device, but I've tried the same config and it doesn't work for me either.

I was able to fix this issue by adding hint.descrtiption attribute into the custom upmix_20to51 pcm device. After that modification new pcm device appeared in the list of aplay -L and snapclient is able to use it now.