Transparency Bug

Im trying to create gifs into jimp images then back into gifs. It works fine on some gifs but on some i get this weird bug: https://imgur.com/a/IlRYSLG

This is how I'm doing it:
```js
GifUtil.read(file).then(inputGif => {
inputGif.frames.forEach((frame, i) => {
// create a Jimp containing a clone of the frame bitmap
jimpCopied = GifUtil.copyAsJimp(Jimp, frame);

Promise.all([jimpCopied, maskP]).then(function(images) {
var lenna = images[0];
//var mask = images[1];

//lenna
//.greyscale()
//.mask(mask, 0, 0);
const fCopied = new GifFrame(new BitmapImage(lenna.bitmap))
frames.push(fCopied);
GifUtil.write("my-creation.gif", frames, {
loops: 0
}).then(gif => {
console.log("written");
});

});

})

})
```

Sorry if its some limitation i missed.