Why is this an issue?

A value that is incremented or decremented and then not stored is at best wasted code and at worst a bug.

Noncompliant code example

fun pickNumber() : Int {
  var i = 0
  var j = 0

  i = i++ // Noncompliant; i is still zero

  return j++ // Noncompliant; 0 returned
}

Compliant solution

fun pickNumber() : Int {
  var i = 0
  var j = 0

  i++
  return ++j
}