I don't know all the ins and outs of how this works, and there's probably a ton of case law that governs when exactly a retrial is allowed following a mistrial (ie how the manifest necessity standard is applied in practice). My understanding is that is that there's a presumption against allowing a retrial due to the constitutional status of the double jeopardy prohibition. Most of the exceptions I'm aware of either involve the jury not being able to reach a verdict or shenanigans from the defense, since you don't want to reward the defense for sabotaging the trial by barring a retrial.

The other thing is (and again I'm not totally sure of this) they might not have to make an official finding of prosecutorial misconduct to bar a retrial. It may be that the standards for declaring prosecutorial misconduct are higher than those needed to bar a retrial, since a finding of prosecutorial misconduct can lead to all sorts of nasty consequences for the prosecutors. It might be enough to say the defense wasn't at fault so they wouldn't be rewarding bad behavior from that side by barring another prosecution.