Sorry for the long delay. This is definitely a bug, and it is not yet reported. I'll send something in to the development team, though there are many bugs with editable thing templates in the extension SDK. Right now we recommend building the templates in ThingWorx, and then exporting them and including them in the SDK as xmls.
I hope this helps!
Just to update this thread with Jira which has already been filed for this issue, please visit this article for more detail : https://support.ptc.com/appserver/cs/view/solution.jsp?n=CS246987&lang=en_US
In short, having isEditableExtensionObject= true for a Java-based extension ThingTemplate is not a supported use case and the ThingWorx Extension Guide will be updated accordingly.