This is happening because you should use the Terminate Contract functionality to close the contract, rather than entering an end date directly.
To do this, go:
Go to Payments - Contracts - Provider Contracts menu option in Admin.
Right-click on the contract you want to terminate.
Select the Terminate option.
Note: If you’ve imported service orders into Admin that are linked to the contract and they’re still open, you won’t be able to terminate the contract.