When performing a callout using External Credential and the Headers are being set in the custom headers section of the External credential, the getheader() method will return a null value in the system debug statements logs even when the headers are being set in the External credential.
This is an expected behavior. We set custom headers configured in external credentials when we're about to send a request to an endpoint. They're not printed in Apex.
The custom headers set in external credentials are not printed in debug statements.
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