Ears for detail

Musescore plugin does not refer to virtual instruments (musesounds, VSTi, soundfont).
Musicians visit Plugin forum here to chat about coding, they may not have experience in virtual instruments at all, see
https://musescore.org/en/handbook/3/plugins
https://musescore.org/en/handbook/4/plugins

To use virtual instruments (musesounds, VSTi, soundfont) use Mixer
https://musescore.org/en/handbook/3/soundfonts-and-sfz-files
https://musescore.org/en/handbook/4/mixer
Issue related to virtual instruments could be seen by many more volunteer helpers when (re)posted on the Support or General or Playback forum https://musescore.org/en/forum
You could also try searching for relevant problems and possible answers see https://musescore.org/en/support
For problems related to Musesounds, you could also contact Muse Hub support https://musehub.zendesk.com/