Alpha channel glitchy when using webp.Decode

[AliFlux]

Hi.

I'm trying to read some webp files and creating an alpha composite using Draw method with Over as a parameter (code below).
The output shows glitchy lines where the alpha channel is merged:

This code was used:

package main

import (
	"fmt"
	"image"
	"image/draw"
	"image/png"
	"log"
	"os"
	"time"

	// swap between these two. The glitch is only in this lib
	//"golang.org/x/image/webp"
	"github.com/chai2010/webp"
)

func getTile(path string) image.Image {
	tileStream, _ := os.Open(path)
	tile, _ := webp.Decode(tileStream)
	defer tileStream.Close()
	return tile
}

func main() {
	tileA := getTile("a.webp")
	tileB := getTile("b.webp")

	canvas := image.NewRGBA(image.Rect(0, 0, 500, 500))

	draw.Draw(canvas, image.Rect(0, 0, tileA.Bounds().Max.X, tileA.Bounds().Max.Y), tileA, image.ZP, draw.Src)
	draw.Draw(canvas, image.Rect(0, 75, tileB.Bounds().Max.X, 75+tileB.Bounds().Max.Y), tileB, image.ZP, draw.Over)

	outputStream, _ := os.Create("result.png")
	png.Encode(outputStream, canvas)

	defer outputStream.Close()
}

Using golang.org/x/image/webp fixes the issue, but it has a couple other problems therefore I can't use it.

The source files are in the following zip:
experiments.zip