Scala variable resets after for-each loop / string gets truncated

I am doing a Spark project. In the following code, I have a string which I use to collect my results in order to write to a file later on (I know this is not the correct way, I am just checking what is inside a Tuple3 returned by a method). The string gets truncated after a for each loop. Here is the relevant part of my code:

val newLine = sys.props("line.separator") // also tried "\n". I am using OS X.

var str = s"*** ${newLine}"

for (tuple3 <- ArrayOfTuple3s) {
  for (list <- tuple3._3) {
    for (strItem <- list) {
      str += s"${strItem}, "
    }
    str += s"${newLine}"
  }
  str += s"${newLine}"
  println(tempStr)
}

print("str=" + str)

The first println method call prints the correct value of the string (the concatenated result), but when the loop ends, the value of str is *** (the same value assigned to it before the first loop).

Edit: I replaced the str immutable String object with a StringBuilder, but no change in the result:

val newLine: String = sys.props("line.separator")

var str1: StringBuilder = new StringBuilder(15000)

for (tuple3 <- ArrayOfTuple3s) {
  for (list <- tuple3._3) {
    for (str <- list) {
      str1.append(s"${str}, ")
    }
    str1.append(s"${newLine}")
  }
  str1.append(s"${newLine}")
  println(str1.toString())
}

print("resulting str1=" + str1.toString())

Edit 2: I mapped the RDD to take the Tuple3's third field directly. This field itself is an RDD of Arrays of Lists. I changed the code accordingly, but I am still getting the same result (the resulting string is empty, although inside the for loop it is not).

val rddOfArraysOfLists = getArrayOfTuple3s(mainRdd).map(_._3)

for (arrayOfLists <- rddOfArraysOfLists) {
  for (list <- arrayOfLists) {
    for (field <- list) {
      str1.append(s"${field}, ")
    }
    str1.append(" -- ")
  }
  str1.append(s"${newLine}")
  println(str1.toString())
}

Edit 4: I think the problem is not with strings at all. There seems to be a problem with all types of variables.

var count = 0

for (arrayOfLists <- myArray) {
  count = arrayOfLists.last(3).toInt
  println(s"count=$count")
}

println(s"count=$count")

The value is non-zero inside the loop, but it is 0 outside the loop. Any idea?

Edit 5: I cannot publish the whole code (due to confidentiality restrictions), but here is the major part of it. If it matters, I am running Spark on my local machine in Intellij Idea (for debugging).

System.setProperty("spark.cores.max", "8")
System.setProperty("spark.executor.memory", "15g")    
val sc = new SparkContext("local", getClass.getName)            
val samReg = sc.objectFile[Sample](sampleLocation, 200).distinct

val samples = samReg.filter(f => f.uuid == "dce03545e8034242").sortBy(_.time).cache()

val top3Samples = samples.take(3)
for (sample <- top3Samples) {
  print("sample: ")
  println(s"uuid=${sample.uuid}, time=${sample.time}, model=${sample.model}")
}

val firstTimeStamp = samples.first.time
val targetTime = firstTimeStamp + 2592000 // + 1 month in seconds (samples during the first month)

val rddOfArrayOfSamples = getCountsRdd(samples.filter(_.time <= targetTime)).map(_._1).cache()
// Due to confidentiality matters, I cannot reveal the code, 
// but here is a description:
// I have an array of samples. Each sample has a few String fields 
// and is represented by a List[String]
// The above RDD is of the type RDD[Array[List[String]]]. 
// It contains only a single array of samples
// (because I passed a filtered set of samples to the function), 
// but it may contain more.
// The fourth field of each sample (list) is an increasing number (count)

println(s"number of arrays in the RDD: ${rddOfArrayOfSamples.count()}")

var maxCount = 0
for (arrayOfLists <- rddOfArrayOfSamples) {
  println(s"Last item of the array (a list)=${arrayOfLists.last}")
  maxCount = arrayOfLists.last(3).toInt
  println(s"maxCount=${maxCount}")
}
println(s"maxCount=${maxCount}")

The output:

sample: uuid=dce03545e8034242, time=1360037324, model=Nexus 4

sample: uuid=dce03545e8034242, time=1360037424, model=Nexus 4

sample: uuid=dce03545e8034242, time=1360037544, model=Nexus 4

number of arrays in the RDD: 1

Last item of the array (a list)=List(dce03545e8034242, Nexus 4, 1362628767, 32, 2089, 0.97, 0.15999999999999992, 0)

maxCount=32

maxCount=0