So , I had called United 1K desk to ask about PZ0 on every flight I had searched , was looking for flights at the end of March , beginning of April from multiple US locations to FRA. Their response is , that they are currently reconfiguring planes for Premium Plus and many of the new planes will lose Business seats , and will be become PP , so they do not want to offer those seats as there could be a plane change

Not sure if that is an amazingly crafted story to appease me or there is some truth to it , but wanted to share the latest reason for PZ0