A DESCRIPTION OF THE PROBLEM :
com.sun.javafx.fxml.BeanAdapter#getClassMethodCache is not checking for interface default methods.

Other people already stumbled across this, too:

https://stackoverflow.com/questions/40360371/are-default-methods-recognized-by-fxml-and-or-scene-builder
https://stackoverflow.com/questions/45641131/javafx-call-default-method-in-scenebuilder-fxml

Implementing this request would allow a large range of mixin-like improvements many projects.

E.g.:
public interface TextProperty
{
StringProperty textProperty();

default void setText(final String text)
{
textProperty().set(text);
}

default String getText()
{
return textProperty().get();
}
}