Unreleased Memory on Property get for Shape.OnAction

Hello,
I've got a macro reading a shape's OnAction property . It's looping, and I saw that Excel keep a lot of memory without release it.

Here how to reproduce the bug.
Create a workbook with one shape.
Create a vba module with this code:

Option Explicit

Public Sub testMem()
Dim strOA As String

While True
  strOA = Sheets(1).Shapes(1).OnAction
  'strOA = Sheets(1).Shapes(1).DrawingObject.Caption
  'strOA = Sheets(1).Shapes(1).DrawingObject.Text
  DoEvents
Wend
End Sub

Run this macro and see Excel memory width in windows task manager.

This occur only when reading this property not on writing, nor on reading other string property.

OS Microsoft Windows*7 Enterprise (Like on XP SP3)
Version 6.1.7601 Service Pack 1 Build 7601
Microsoft Excel 2010 (14.0.6129.5000) SP1 (32bits)