Wrong white background when converting SVG to PNG with gradient - php

Question

I want to convert SVG images to PNG keeping transparency. In the case of solid color shapes works fine, but when the shape is filled with a gradient, then the output image renders an unexpected solid white background.
I want some solution using Imagick library, but some code to preproccess svg code is also acceptable.
Here a minimal example of the issue:
<?php
$svg = <<<SVG
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<svg width="100" height="100">
<ellipse style="fill:red" cx="50" cy="50" rx="50" ry="50" />
</svg>
SVG;
$img = new \Imagick();
$img->readImageBlob($svg);
$img->setImageBackgroundColor('transparent');
//IMAGE WITH NICE TRANSPARENT BACKGROUND
$img->writeImage(__DIR__ . '/circle_plain.png');
$svg = <<<SVG
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<svg width="100" height="100">
<defs>
<linearGradient
id="linearGradient1">
<stop
style="stop-color:#ff0000;stop-opacity:1;"
offset="0"/>
<stop
style="stop-color:#00ff00;stop-opacity:1;"
offset="1"/>
</linearGradient>
<linearGradient
xlink:href="#linearGradient1"
id="linearGradient2"
x1="0"
y1="50"
x2="100"
y2="50"
gradientUnits="userSpaceOnUse" />
</defs>
<ellipse style="fill:url(#linearGradient2)" cx="50" cy="50" rx="50" ry="50" />
</svg>
SVG;
$img = new \Imagick();
$img->readImageBlob($svg);
$img->setImageBackgroundColor('transparent');
//IMAGE WITH WRONG WHITE BACKGROUND!!
$img->writeImage(__DIR__ . '/circle_gradient.png');