--> I think there is a problem. <--
I never see this issue until now.
Testing your "private build" attatched to previous forum message, concatenating 2 video and exporting all as "lossless", after after a reasonable conversion time the output file is NOT a valid video.
VLC can open it but the video is blank!
You can do the fastforward/fastbackward tracking of video, the file itself is readed as "video file" and not simple garbage, but VLC doesn't show anything. Black video. No sound.
Matteo