I have a project subobject in v14.2 and have created a parent object attribute for that
I can not see it in the available columns for create view the same way as you
However I can see it for the edit view and list view
Thinking about it when you are creating a new suboject instance it will be for the current master object instance and the current value of the master object attribute will be the noneditable value of the subobject attribute.
If is not editable and automatic why would you need it in the create view.
I think it is working as designed.