The issue is, with ffmpeg identifying the mjpeg stream as a vidoe stream when MCEBuddy tries to remux the wtv file, ffmpeg tries to copy the mjpeg stream also (since it's identified as a video stream) and the remux fails, i.e. mcebuddy is dead right there since remuxing is fundamental.
Logically also it doesn't make sense to mark a mjpeg stream as a video stream, so there's a bit apparntly which identifies this isssue, and the developer (PEter Ross) needs to add some code to recognize this bit and mark the stream as an attachment and not video so that when ffmpeg tries to copy the video streams (map 0:v) it only copies the actualy video and not the mjpeg.
Logically also it doesn't make sense to mark a mjpeg stream as a video stream, so there's a bit apparntly which identifies this isssue, and the developer (PEter Ross) needs to add some code to recognize this bit and mark the stream as an attachment and not video so that when ffmpeg tries to copy the video streams (map 0:v) it only copies the actualy video and not the mjpeg.