Personal experience: that may because ( for not able to download video on youtube)

They (Google) have adapted a kind of new video technology called AV1 codec, as I know some of the downloaders/ devices do not support this kind of codec, hence that became the reason why they failed to download/ decode their downloaded video.

There are some useful resources may be helpful.

https://bloggeek.me/av1-video-codec/

https://www.winxdvd.com/video-converter/a-brief-intro-to-av1-codec.htm