As wielder of an elder wand that belonged to Harry, during Voldemort’s Last Stand, there are two instances of Priori Incantatem occurring, back to back.
Basically, the current order of events makes total sense. Since Nagini was the last horcrux, when the body died, the fragment of Voldemort’s soul locked in it was released. This disrupted the first instance of Priori Incantatem, and Voldemort was now significantly weaker than Harry – so Harry won. Naturally, the Elder Wand’s killing spell was reflected back on Voldemort because of Priori Incantatem outcome.
But, if Voldemort was just told by Harry that the Elder Wand wouldn’t work against Harry because it belonged to Harry, and was a wand that would win any duel for its master, why would Voldemort willingly try and continue to use it? This is doubly so, since after he had cast the killing spell on Harry once it had only torn Voldemort’s soul from Harry’s body – wouldn’t that even imply that the Elder Wand would realistically refuse almost any attempted direct harm against its owner?
Also, wouldn’t a first refusal of the Elder Wand to kill it’s own master also imply an additional refusal even if Voldemort had won either of the Priori Incantatem matches (in which case, the worst that could happen in this line of thinking is, Voldemort has Harry’s wand since Harry’s spell is Expelliarmus)?