We have not seen this before. My guess is it could be the way the videos were encoded.

If the video(s) are being transcoded on-the-fly by Mezzmo, then they may be being re-muxed for fast transcoding speed. This means that the video and/or audio codecs/channels in the original video are copied into the transcoded video. If you wish, you can pre-transcode the video and do a full transcode of the codecs/channels. This may eliminate any possible encoding issues in the original video. When pre-transcoding, make sure you select the 'Force full transcode of all channels' checkbox on the Pre-transcode Formats dialog. Let us know how it goes.