In The Cursed Child, why didn't Harry's blood in Voldemort's body keep Harry alive?

In The Cursed Child, why didn’t Harry’s blood in Voldemort’s body keep Harry alive in the alternate timeline in which Cedric kills Neville, which meant that Nagini stays alive, and Voldemort wins the war?

This is written in Harry Potter and the Deathly Hallows, Chapter 35 (King’s Cross):

“He took your blood believing it would strengthen him. He took into his body a tiny part of the enchantment your mother laid upon you when she died for you. His body keeps her sacrifice alive, and while that enchantment survives, so do you and so does Voldemort’s one last hope for himself.”

J.K. Rowling stated this on the F.A.Q. on her old website – the blood in the Dark Lord acted like a Horcrux for Harry and the blood in him kept the sacrifice (and therefore Harry) alive while he was:

Having taken Harry’s blood into himself, Voldemort is keeping alive Lily’s protective power over Harry. So Voldemort himself acts almost like a Horcrux for Harry – except that the power of Lily’s sacrifice is a positive force that not only continues to tether Harry to life, but gives Voldemort himself one last chance (Dumbledore refers to this last hope in chapter 35). Voldemort has unwittingly put a few drops of goodness back inside himself; if he had repented, he could have been healed more deeply than anyone would have supposed. But, of course, he refused to feel remorse.

I haven’t seen an answer from J.K. Rowling regarding this question yet?