I learn French on my own, I do the exercises from "Grammaire-Cours de Civilisation française de la Sorbonne- 350 exercices, Niveau Superieur I".
In that book, there is an exercise as follows
Employer, si nécessaire, les articles qui conviennent.
Je ne bois pas... vin, mais.... jus de pomme
I knew that des, de la, du become de in negative sentences (or before an adjective followed by a noun, or in a sentence in which être is involved) then I decided to write that sentence as
Je ne bois pas de vin, mais de jus de pomme.
But the answer that book gives is
Je ne bois pas du vin, mais du jus de pomme.
So, in this negative sentence, du doesn't become de.
My question is: Why? Is it the case that in a complex sentence des, du, or de la doesn't become de?