Nativescript - animation on a button

I've got a button and I would like it to animate it - fade in and move up.

    @keyframes ios_about_button
        from { opacity: 0; transform: translate(0,40) }
        to { opacity: 1; transform: translate(0,0) }
        animation-name: ios_about_button;
        animation-duration: 0.4s;
        animation-delay: 3s;
        animation-fill-mode: forwards;

<Button id="buy" class="bttn-style" text="BUY" tap="{{ onSubscribeThisApp }}" opacity="0" />

Above is the css and xml code.

It works as expected - note the opacity="0" in the xml - trying to achieve a fade-in.

The problem is that when I tap on the button - the animation runs again.

Not sure what's going on...