Hi Sue,
I am afraid, I was not able to see your problem in TreeViewAdv. I checked in the latest build 4.1 Beta version release.Please take a look at this sample which reproduces your problem.
In this AfterCheck event is fired only when the ShowCheckBoxes is set to true.
Thanks for your patience and your continued interest in Syncfusion products.
Best Regards ,
Deepa.T.S
Aftercheck.zip