D&D a TitledPane.
Look at the Expanded property in the Inspector - it is set to true.
Save the FXML.
Open the FXML in a text editor - bug - there is no mention of the expanded property.
Now switch back to SB, set the expanded property to false and save.
Switch back to your text editor - you will now see that 'expanded="false"' is saved in the FXML.

This also impacts TitledPane's inside an Accordion.