Qualified immunity for the feds is such an odd concept.
Though the 8th Circuit conceded that Weyker's sex-traffing investigation was "plagued with problems from the start"—the trial judge found, for example, that she fabricated information and lied multiple times under oath—the court said she was, in fact, immune. But that wasn't because she was entitled to qualified immunity. Rather, although Weyker was a St. Paul police officer, she had been cross-deputized on a federal task force to carry out the investigation, making available to her the legal protections afforded to federal law enforcement—a much higher bar for alleged victims to clear.
But I guess if you are the law, you're automatically above the law... ugh.