Bottle of Cachaça 51 tasting like sour milk?
Further research on this revealed my own answer: Bagasse is the waste product from extracting juice of sugar cane, and if naturally fermented/decaying, smells like sour milk!

Depending on the grade of sugar cane juice (freshness of sugar cane itself, as well as heads vs tails of extraction), the juice can have some of the bad flavour from the bagasse, and it's sufficiently volatile to persist in the distillation process!