Remux and Stitching videos

You can't use the Remux presets while stitching videos is enabled.

Stitching videos together isn't an easy task, as you can't just simply write the two videos files one after the other. They need to be transcoded to a format that can be stitched this way - MPEG-2. As remux presets only copy over the source video and audio tracks, the Remux preset will attempt to copy over MPEG-2 video, which is not supported by all containers, mainly it's not supported in MP4 containers.

This applies also to presets, where you choose to Copy the track in preferences.