When the user types a new value into a ComboBox, the new value is apparently not available (to getValue()) until the user types Enter, an action that the UI gives absolutely no clue might be required. You can't possibly expect end users to be familiar with the JavaFX documentation that says "when the user commits textual input into the textfield (commonly by pressing the Enter keyboard key), the value property will be updated."

At the very least, the value should be updated when focus leaves the ComboBox.