alphadecodebin: incorrect rendering on lossless VP9

I found that VP9 with alpha channel encoded on lossless mode is not decoded correctly by both GStreamer on master branch and FFmpeg as well, but at the same time it is rendered correctly by Firefox and Chromium, so there must be something GStreamer can do about it.

Visually it looks like if alpha channel is being effectively ignored in this case.

Example file: https://cdn.streamelements.com/uploads/187c102b-ac08-4843-8ac9-4e6be93688b9.webm