I thought the point about "beneficial mutations" was that they served a purpose to help the host survive. If the mutation came about which caused the loss of ability to synthesize ascorbic acid then this would reduce the viability of the host. It only reduces the viability if there is inadequate ascorbic acid in the diet. Think of cave fish. Normally, any mutation that caused loss of vision would be very bad. But in a cave it's neutral, neither selected for nor against.
"It only reduces the viability if there is inadequate ascorbic acid in the diet. "
That was my point.