YouTube delivers the video and audio in different streams. That has no effect on playing the videos because NewPipe's embedded player (ExoPlayer) merges both streams while playing a video.
In contrast, our download library does not have the ability to merge both streams. This process would therefore require another extra library. Due to the size of the app, we have refrained from working on a concrete solution. Nevertheless, the entire download process must be refactored. It is therefore possible that a good solution to this problem will emerge.