why wouldnt it result in a mistrial?
Because the law doesn't require a prosecutor to present exculpatory evidence to a Grand Jury. The Defense isn't even involved, just a prosecutor selling his case with no rebuttal. This is why they say "you could indict a ham sandwich". The evidence was not withheld from the defense during the trial.
I can't believe, even in Austin, that there wasn't one person on the jury that refused to convict this man.