Same as JENKINS-43032, except the value I bound to a username in withCredentials is being bound to a string not related to the binding.  

ex.

CHART_NAME=ingestion-tracking

value of DATASOURCE_USERNAME = ingestion

withCredentials([usernamePassword(credentialsId: "${DATASOURCE_CREDS}", passwordVariable: 'DATASOURCE_PASSWORD', usernameVariable: 'DATASOURCE_USERNAME')
]) {
  sh "echo ${DATASOURCE_USERNAME}"

  sh "echo ${CHART_NAME}"
}

will yield:
****
****-tracking

I expect:
****

ingestion-tracking