ADDITIONAL SYSTEM INFORMATION :
Windwos 10
java version "1.8.0_181"
Java(TM) SE Runtime Environment (build 1.8.0_181-b13)
Java HotSpot(TM) 64-Bit Server VM (build 25.181-b13, mixed mode)

A DESCRIPTION OF THE PROBLEM :
If i use more than one style class in the FXML file for a spinner, just the first class is loaded.
This is independent of using attribute (e.g. <Spinner styleClass="class1, class2") or nodes (e.g. <Spinner><styleClass><String fx:value="class1"/>...)
Other controlls (e.g. TextField, Combobox) seems to work fine.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
assigned style classes to Spinner: ["font-larger", "font-bold"]
ACTUAL -
assigned style classes to Spinner: "font-larger"

---------- BEGIN SOURCE ----------
example.css
.font-larger {
    -fx-font-size: 20;
}
.font-bold {
    -fx-font-weight: bold;
}


example.fxml
<VBox xmlns="http://javafx.com/javafx/8.0.181" xmlns:fx="http://javafx.com/fxml/1">
  <children>
    <TextField>
      <styleClass>
        <String fx:value="font-larger"/>
        <String fx:value="font-bold"/>
      </styleClass>
    </TextField>
    <Spinner fx:id="spinner" editable="true" min="0" max="100" initialValue="12">
      <styleClass>
        <String fx:value="font-larger"/>
        <String fx:value="font-bold"/>
      </styleClass>
    </Spinner>
  </children>
</VBox>

java class
public class SpinnerStyleClass extends Application {

public static void main(String[] args) {
Application.launch(args);
}

@Override
public void start(Stage primaryStage) throws Exception {
FXMLLoader loader = new FXMLLoader(Thread.currentThread().getContextClassLoader().getResource("example.fxml"));
Parent root = loader.load();

Scene scene = new Scene(root);
scene.getStylesheets().add(Thread.currentThread().getContextClassLoader().getResource("example.css").toExternalForm());

// expected result ["font-larger", "font-bold"]
assert root.lookup("#spinner").getStyleClass().size() == 2;

primaryStage.setScene(scene);
primaryStage.show();
}
}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Wrap String values in ObservableArrayList:

    <Spinner fx:id="spinner" editable="true" min="0" max="100" initialValue="12">
      <styleClass>
        <FXCollections fx:factory="observableArrayList">
          <String fx:value="font-larger"/>
          <String fx:value="font-bold"/>
        </FXCollections>
      </styleClass>
    </Spinner>


FREQUENCY : always