Responses say Botox cosmetic is not responsible for metallic taste. Why not?

Bell's Palsy in April, now synkinesia. Facial muscles permanently contracted. 1st symptoms of BP was "burning" sensation in mouth. After DX, rapid advance to continuous metallic taste. Botox reduced some tightness. Sense of taste also changed; not better or worse, different. Responses say Botox cosmetic not responsible for metallic taste. Why not? If Botox blocks messages to nerves, could it not also block messages to the facial nerve affected by the BP& responsible for the disturbance in taste?