In my experience, which is not definitive, QI can be denied in cases where the government employee exceeds their authority or deviates from established agency procedure in a manner that rises to negligence. The definition of any of the above is determined by the court.

For example, if the agency says "you shall not drive a government vehicle for 8 hours after consuming alcohol", the employee drinks and then responds to a work call-out in a company vehicle and causes a bodily injury accident. The court may hold that QI did not apply to the case as the employee violated established procedure.

It is my opinion (and only my opinion) that one reason that TSA will not disclose the SOP (keeps it SSI) is to avoid lawsuits against screeners and/or the agency since a plaintiff is not entitled to know the procedure. If that is the case, it is contrary to established rules for classification of information, but those rules are merely speed bumps for determined government actors.