No Subscription Start Date Option showing when creating a new plan

With Version 3,1.0 of MP (5th July 2022) , it introduced a new option 'Subscription Start Date'
All my existing plans have that option in them. These plans were already in place before we upgraded to verion 3.1.0

I am now trying to create a new plan where I want to use this option. However, when I create the new plan the Subscription Start date is not there????
I have attached 2 screen shots. one shows one of my existing plans which has the option and a new plan which does not.
I can't see why it does not show. I can't remember having to set anything up to make these options show.....any ideas??
Here is existing plan with the option
 
Here is new plan without the option