[Nebullis]

If the mutation came about which caused the loss of ability to synthesize ascorbic acid then this would reduce the viability of the host. Doesn't this go against evolutionary theory and the idea of beneficial mutations?

Beneficial is a subjective term. Does this mutation cause the human species to breed less? This mutation is a loss of function but doesn't impact the fecundity of human species, which is the only thing that natural selection cares about. It is one of the simple examples where we can point to a common origin of a specific mutation.

[webstersII]

"Beneficial is a subjective term. Does this mutation cause the human species to breed less? This mutation is
a loss of function but doesn't impact the fecundity of human species, which is the only thing that natural
selection cares about. "

I disagree with your point here. This mutation would cause the human species to breed less and survival would be severely impacted unless there was an ample supply of vit. C in their diet.