After further investigating your reported issue our development team found out that this issue is caused by an Advanced Installer limitation. More exactly the issue appear because of the value of the properties (whose name starts with underscore char) are not always preserved from Wizards Dialogs Stage to the Install Execution Stage.
To avoid this limitation you will have to rename your property name from:
We apologize for this inconvenience.
Let us know if this helped.
All the best,