Upon Daniel's suggestion on IRC ("ok, so you probably want to look at /usr/share/pulseaudio/alsa-mixer/paths/analog-output*", "IF it turns out not to be a codec (driver) issue, you may try changing the volume = merge") I tried replacing all occurrences of "merge" with "ignore" in /usr/share/pulseaudio/alsa-mixer/paths/analog-output.conf. This stops the behavior described in the bug, and I can set the volumes however I want in alsamixer.
Upon Daniel's suggestion on IRC ("ok, so you probably want to look at /usr/share/ pulseaudio/ alsa-mixer/ paths/analog- output* ", "IF it turns out not to be a codec (driver) issue, you may try changing the volume = merge") I tried replacing all occurrences of "merge" with "ignore" in /usr/share/ pulseaudio/ alsa-mixer/ paths/analog- output. conf. This stops the behavior described in the bug, and I can set the volumes however I want in alsamixer.