Problem occurring in Ardour with VST.
The CC 121 parameter is automated during an initialization step.
This resets sfizz's CC to zero values, including volumes, and the instrument is silent.

There are really 2 problems here

• sfizz's reset function should not send zero, but instead the initial values according to the SFZ file
• the VST cannot properly restore its settings

I vaguely recall about some VST documentation, which suggests that there shouldn't be a parameter automation happening as a result of another. I deduce that VST should not be processing reset-all-controllers at all.
In LV2, where the plugin's CC are host-controlled, it's probably safe to assume the same.