When defining multiple usernamePassword pairs to be set from credentials only the last one is actually configured in the job.

wrappers {
    credentialsBinding {
        usernamePassword("USER1", "PASS1", "credential1")
        usernamePassword("USER2", "PASS2", "credential2")
    }
}

Since the UI when manually configuring a job offers the ability to add multiple items I would expect all of them to be configured in the job, not just the last one.