Why are commitments not reversed when invoices are under paid?

Summary:

Why are commitments not reversed when invoices are under paid?

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We created a PO for 100 USD. And a commitment of 100 USD was created in GL for the charge account.

We created a matched invoice for the same PO of 80 USD and closed it. And paid the invoice.

When we checked the commitments, it was not reduced by 20. Is it expected ?

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