STEPS TO REPRODUCE:
1. Create an Entity, assign Fiscal Time Periods to the Entity (including dates prior to the Freeze Date you plan to use), and associate the Entity with an OBS.
2. On the Entity go to the "Plan Defaults" tab and select a date for "Freeze Date"
3. Create a new Project, assign the OBS to the Project that is linked to the Entity.
4. Create a new Cost Plan for the Project.
5. On the Cost Plan details you will see Time Periods prior to the Freeze Date.
In the Modern UX, you can assign values to these fields. In the Classic Interface it will not allow this, and it respects the Freeze Date.
You should not be able to assign values to items prior to the freeze date. It should work as it does in the Classic interface.