WebEngine fails to renders canvas elements when using software renderer

I use WebEngine based browser as a GUI for my application. I use software Qt Quick backend to avoid OpenGL dependency. When the html page produced by the application is complex enough the browser may render some parts of the page upside-down or as a black rectangle.

I've tried Qt 5.8.0, 5.9.0 and 5.9.1 - they all are affected.

The screens below are produced with simplebrowser from Qt 5.9.1 Examples.

When using OpenGL renderer everything is fine:

When using software renderer (QMLSCENE_DEVICE=softwarecontext ./simplebrowser) some part of the page is drawn upside-down while mouse events act as usual: