As per the docs, setting sourceSize.height on an Image element to a value while leaving sourceSize.width at 0 (or vice versa) should scale the image such that the configured height is used and the other value is scaled according to the image's aspect ratio. And this is also the behavior seen on Qt 5.9 and below. But with Qt 5.10, this no longer works. Leaving one component of sourceSize at 0 causes the image to not be shown at all.
See attached test case project. When this run on Qt 5.9, it shows the help icon on the right. On Qt 5.10 it doesn't.