The case involved two prisoners who had been found innocent and released from jail and who filed a lawsuit against the prosecutor. So why didn't the Court issue a decision? The parties settled, for $12 million.
Here's a link to articles about the case. And here's a link to the case: Pottawattamie County v. McGhee at OYEZ.
Do you think the U.S. Supreme Court would have found the prosecutor was not entitled to immunity? Judges systemically give immunity to prosecutors, judges, and elected officials when lawsuits are filed against them... even when it's obvious those individuals have violated the constitutional rights of others.