I would guess that the GPU simply cannot decode 4:2:2 JPEGs.
I'm not even sure that it supports decoding MJPEG at all as I don't see a MJPEG entry point. Some MJPEGs have different headers for the JPEGs so that it's not even a valid JPEG on its own.
In addition to that I guess that the GPU just gets fed the direct video bitstream via the VA API. Even if the MJPEG only contains baseline JPEGs (the only ones supported) I doubt that a standard JPEG decoder could decode a MJPEG bitstream.
I'd just use software decode as I don't think that a GPU decode would be noticable faster/efficient for JPEGs on this CPU.
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