Thanks Francisco. I hadn't seen the Full Parameters List in Mission Planner for a while and assumed it had been removed. Clearly my mistake. Following your message, I looked for it again and found the way to access it by setting Mission Planner Layout to "Advanced". I can now manually configure the RNGFND_PIN to 14 as I had done in the past. Thanks!.
Is there any reason why pins 13 and 14 are not selectable options in the dropdown menu for RNGFND_PIN?