Hi there,i also had these sound problems using a TOSLink to my external receiver. Today i compiled the new 1.2.6 packages of libasound2 and libasound2-plugins for bullseye and the 5.1 sound is working again.
I use the default 60-a52-encoder.conf fiĺe. -- MfG, Christian Welzel GPG-Key: pub 4096R/5117E119 2011-09-19 Fingerprint: 3688 337C 0D3E 3725 94EC E401 8D52 CDE9 5117 E119