JPEG XR export w/ transparency wildly oversized

[foxyshadis]

I was testing converting a graphic to JPEG XR, and I just could not get it below a certain size, even at zero quality. I eventually figured it might be the transparency, but try as I might, I could not find any way to flatten the image with a white background (even when it looks like it's now white); finally I did it in Photoshop. The file size differences were enormous.

Original with transparency
Original without transparency

JXR with transparency, q0, 213K
JXR without transparency, q0, 29K

An order of magnitude difference indicates that something's wrong with compressing the alpha channel.

[xnview]

you means that alpha channel take a lot of space when compressing in jpegxr?