Why doesn’t changes made to some of my properties don’t get saved in the aspx file during design time?

Nested properties in your controls should be marked with the NotifyParentProperty(true) attribute in order for the designer to get notified that it’s value has changed.

For example, the following property (SubProperty) declaration should include the above mentioned attribute, for the designer to persist changes to that property during design-time:

myControl.SomeComplexProperty.SubProperty = 'some value';