The opacity property of a custom button, that is initially disabled, is not updated once it has been enabled. The following conditions needs to be met to reproduce the issue:

The custom skin on the button is set using -fx-skin via css stylesheet
The custom button is a part of a layout which has replaced the original root of the Scene
The bug is reproducible with JavaFX 10, 11, 12 but is not reproducible on JDK 9, which is why it seems to be a regression.

It seems to be related to https://bugs.openjdk.java.net/browse/JDK-8201285 and to the issues marked as duplicates / related in the said issue.

REPRODUCIBILITY: This bug can be reproduced always.

SCSS
CustomButtonSkin.java
import javafx.scene.control.Button;
import javafx.scene.control.skin.ButtonSkin;

public class CustomButtonSkin extends ButtonSkin {

    public CustomButtonSkin(Button button) {
        super(button);
    }
}
Main.java
import javafx.application.Application;
import javafx.geometry.Pos;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.BorderPane;
import javafx.scene.layout.HBox;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

public class Main extends Application {

    @Override
    public void start(Stage primaryStage) {
        BorderPane borderPane = new BorderPane();

        Button button = new Button("Click me!");
        button.setOnAction(e -> {
            Button disabled = new Button("Disabled");
            disabled.setDisable(true);
            Button enable = new Button("Enable");
            enable.setOnAction(ev -> disabled.setDisable(! disabled.isDisable()));
            final HBox root = new HBox(5, enable, disabled);
            root.setAlignment(Pos.CENTER);
            root.getStyleClass().setAll("view");
            borderPane.setCenter(root);
        });

        VBox root = new VBox(20, button);
        root.setAlignment(Pos.CENTER);

        borderPane.setCenter(root);

        final Scene scene = new Scene(borderPane, 300, 275);
        scene.getStylesheets().add(getClass().getResource("custom.css").toExternalForm());
        primaryStage.setScene(scene);
        primaryStage.show();
    }

    public static void main(String[] args) {
        launch(args);
    }
}
custom.css
.view {
}

.button {
    -fx-skin: "sample.CustomButtonSkin";
}