Cost Plan Breakdowns do not delete when it's Cost Plan is deleted.
This is out of box behavior.
When a Cost Plan is created there will be two sets of Breakdowns created. One for the Cost Plan and the one set for the Project itself. Deleting the Cost Plan will delete the Breakdowns associated with the Cost Plan but not the Project.
When cost plan record for a project get deleted, it's child breakdown of type "Requirement" also get deleted.
The breakdown of type "Task" associated with the project gets updated with planned cost, they are NOT supposed to get deleted.
The breakdown of type "requirement" are for Cost Plan entity.
The breakdown of type "task" are for the Task entity referred in task field.
Task typed breakdown are meant for aggregation at parent entity. Task-breakdown are the kind of aggregation records at Project/Demand or Program level, these breakdown store cost by fiscal period by cost type ( opex/capex).
Consider a case where a project has more than a cost plans. If any of the cost plan record get deleted then corresponding amount from these task-tasked breakdown get updated and only requirement typed breakdown specific to deleted cost plan record gets cascade deleted.
For performance optimization we don't regenerate these task-type breakdown every time cost plan record create/update/deleted.