Iterable withIndex behavior discrepancy on Jenkins

withIndex.txt

// Run this in a Jenkins step
String jsonStr = "['zero', 'one', 'two']"
JSONArray jsonArray = new JsonSlurper().parseText(jsonStr) as JSONArray
jsonArray.withIndex().each{Object str, Integer idx -> currentBuild.echo("${idx.toString()} : ${str}")}

// OUTPUT from jenkins
//        [Pipeline] echo
//        null : [zero, 0]
//        [Pipeline] echo
//        null : [one, 1]
//        [Pipeline] echo
//        null : [two, 2]
  
// Run this locally in a groovy file or unit test
String jsonStr = "['zero', 'one', 'two']"
JSONArray jsonArray = new JsonSlurper().parseText(jsonStr) as JSONArray
jsonArray.withIndex().each{Object str, Integer idx -> println("${idx.toString()} : ${str}")}

// OUPUT Local
//        0 : zero
//        1 : one
//        2 : two