Users can enter Inventory Order Allocations after the Allocation End Date when Allow Date Exceptions is set to Prevent.
Allow Date Exceptions are intended to prevent Orders from being submitted if the Order Date is outside of the Order Start Date and Order End Date. It is not used to enforce when allocations may take place or if they are outside of the Allocation Start Date and Allocation End Date.
This is working as designed. A custom Validation Rule can be created to restrict based on the date and Allow Date Exceptions field.
CRM Help Documentation: Inventory Ordering