As title. Below is a test case to reproduce the issue. See comments in the file for different test scenarios I've tried.

package demo.combobox;

import javafx.application.Application;
import javafx.application.Platform;
import javafx.collections.FXCollections;
import javafx.geometry.Insets;
import javafx.scene.Group;
import javafx.scene.Scene;
import javafx.scene.control.ChoiceBox;
import javafx.scene.control.ComboBox;
import javafx.scene.control.Label;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

public class ComboBoxSetItemBug {
    public static class ComboBoxSetItemsBug extends Application {

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

        @Override
        public void start(Stage stage) {
            final ComboBox<String> comboBox = new ComboBox<>();
            comboBox.setValue("Arial");

            final ChoiceBox<String> choiceBox = new ChoiceBox<>();
            choiceBox.setValue("Arial");

            Runnable runnable = new Runnable() {
                public void run() {
                    String value = comboBox.getValue();
                    System.out.println("(ComboBox) value before setItem: " + value);
                    comboBox.setItems(FXCollections.observableArrayList(getFontNames()));
                    System.out.println("(ComboBox) value after setItem: " + comboBox.getValue());

                    // I have to trick it - set the value to something else then set it back to make the "Arial" showing
                    // comboBox.setValue("");
                    // comboBox.setValue(item);
                    // System.out.println("(ComboBox) item after select: " + comboBox.getValue());

                    //////////////////////////////////

                    System.out.println("(ChoiceBox) value before setItems: " + choiceBox.getValue());
                    choiceBox.setItems(FXCollections.observableArrayList(getFontNames()));
                    System.out.println("(ChoiceBox) value after setItems: " + choiceBox.getValue());

                    // with choicebox, the same trick below doesn't work either
                    // choiceBox.setValue("");
                    // choiceBox.setValue(item);
                    // System.out.println("(ChoiceBox) value after select: " + choiceBox.getValue());
                }
            };
            // if i run it directly without using runLater, the text shows just fine.
            // runnable.run();
            Platform.runLater(runnable);

            VBox vbox = new VBox(6, comboBox, choiceBox, new Label("The selected value is Arial but the text is not shown on " +
                    "\nboth ComboBox and ChoiceBox. You can read comment " +
                    "\nin the file for more information"));
            vbox.setPadding(new Insets(10));

            Scene scene = new Scene(new Group());
            stage.setTitle("ComboBox Bug");
            ((Group) scene.getRoot()).getChildren().addAll(vbox);
            stage.setScene(scene);
            stage.show();
        }

        // partial font list
        public static String[] getFontNames() {
            return new String[]{
                    "Agency FB",
                    "Aharoni",
                    "Algerian",
                    "Andalus",
                    "Angsana New",
                    "AngsanaUPC",
                    "Aparajita",
                    "Arabic Typesetting",
                    "Arial",
                    "Arial Black",
                    "Arial Narrow",
                    "Arial Rounded MT Bold",
                    "Arial Unicode MS",
            };
        }
    }
}