Date First Held field remains locked for PHEVs in Quoting Stage

I want the Date First Held field to unlock when quoting a PHEV vehicle so that I can input the date for budget adjustments on existing client vehicles. Currently, even after updating the quote to meet all specified FBT/LCT conditions (Used/Demonstrator, EV Exempt Reportable, LCT 0.00), the Date First Held field remains greyed out and locked in the quoting stage.