Hi Eric,
I have had checked this issue. I am afraid but I was not able to reproduce this issue.
For your convenience, I am attaching a sample file with two buttons. One activates CustomZoom Tool, with explicit declaration of "SingleActionTool = false"; and the other activates the Standard Zoom Tool.
The Tool does not get detached after MouseUp()or Zoom and stays until another tool replaces this.
The sample is here at:
http://www.syncfusion.com/support/user/uploads/Diagram_WF_CustomZoom_c92f3f6a.zipDiagram_WF_CustomZoom.zip