Large memory build-up when modifing object properties using string keys => object["someKey"]=

Assigning a rational number to an object internal value using a string key selector generates garbage. Ex:

var rational: Number = 0.1;

someObject["someValue"] = rational;

My current use case:

I was developing a small custom animation engine ( basically a tweener ). When I tested it with about 400 tweening objects at the same time, I noticed in Scout a large memory build-up ~1-2 MB/s ( NOT A memory leak as it was getting GC'ed). Weird thing was that Scout (or any other profiler for that matter) did not show what objects were created/responsible for the build-up. I spent a day tracking this issue down to the very line that causes this. It seems that setting an object property using a string key like this:
"object["someKey"] = someValue" generates some internal Flash garbage ( not caught by profiling). Doing this repeatedly on every frame becomes noticeable. Every generic object tweening engine I tested suffers from this.
What baffled me was that if the value( "someValue" ) assigned to the objects property was an "int" type
(ex: "object["someKey"] = 5") did not generate garbage(total memory flat line in scout). If it was a rational number then it generated MB/s triggering GC every ~5 secs.

Not sure if it is a flash bug or a "normal" thing (because I think there is some sort of internal lookup for the property inside the object) but I am curious for an explanation. I am pasteing here a minimal code example to reproduce this issue.

Minimal sample code to reproduce this issue:

package

{

  import flash.display.Sprite;

  import flash.events.Event;

  [SWF(width="800", height="480", frameRate="60" , backgroundColor="0x000000")]

  public class Main extends Sprite

  {

  private var objectsArray:Vector.<ValueObject> = new Vector.<ValueObject>;

  private var rational: Number = 0.1;

  private var integer: int = 1;

  private var maxObjectsCount: int = 1000;

  public function Main()

  {

  stage.addEventListener( Event.ENTER_FRAME, update, false,0, true );

  //generate  objects

  for (var i:int=0; i<maxObjectsCount; i++)

  {

  var object: ValueObject = new ValueObject();

  objectsArray.push(object);

  }

  }

  private function update(event: Event ):void

  {

  var i:int;

  var len:int = objectsArray.length;

  for(i=0; i<len;i++)

  {

  var object:ValueObject = objectsArray;

  //this generates garbage

  object["value"] = rational;

  //this doesn't generate garbage

  //object["value"] = integer;

  //this doesn't generate garbage

  //object.value = rational;

  }

  }

  }

}

internal class ValueObject

{

  public var value: Number = 0;

}