A DESCRIPTION OF THE PROBLEM :
JavaFX draws strange pieces in round corners of Tab headers. The problem was also discussed on SO https://stackoverflow.com/questions/71665510/weird-corners-in-tab-headers-in-javafx

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
This is CSS code:

.tab-pane > .tab-header-area > .headers-region > .tab > .tab-container > .tab-label {
    -fx-alignment: center;
    -fx-text-fill: -fx-text-base-color;
    -fx-padding:0 10 0 0;
    -fx-font-size: 15px;
}

.tab-header-area .tab {
    -fx-padding:4 10 5 10;
    -fx-border-radius: 10 10 0 0;
    -fx-background-radius: 10 10 0 0;
}

This is Java code:

public class JavaFxTest1 extends Application {
    public static void main(String[] args) {
        launch(args);
    }

    @Override
    public void start(Stage primaryStage) {
        var tabPane = new TabPane();
        VBox.setVgrow(tabPane, Priority.ALWAYS);
        primaryStage.setTitle("Hello World!");
        Button button = new Button();
        button.setText("NewTab");
        button.setOnAction(new EventHandler<ActionEvent>() {

            @Override
            public void handle(ActionEvent event) {
                Tab tab = new Tab("New tab");
                var content = new VBox();
                tab.setContent(content);
                var splitPane = new SplitPane();
                VBox.setVgrow(splitPane, Priority.ALWAYS);
                content.getChildren().add(splitPane);
                splitPane.getItems().addAll(new TextArea("One two three"), new TextArea("One two three"));
                tabPane.getTabs().add(tab);
            }
        });

        VBox root = new VBox();
        root.getChildren().addAll(tabPane, button);
        var css= this.getClass().getResource("test1.css").toExternalForm();
        var scene = new Scene(root, 800, 600);
        scene.getStylesheets().add(css);
        primaryStage.setScene(scene);
        primaryStage.show();
    }
}

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Round corners of the Tab without any extra pieces.
ACTUAL -
I tested in Windows (Win7, java 18, JavaFX: 19-ea+3):
And in Ubuntu (Ubuntu 20.04.3 LTS, openjdk version "14.0.2" 2020-07-1, JavaFX: 19-ea+3 for linux):

FREQUENCY : often