For example, this invidious instance offers a download option for a YoutTube video, as that instance does for all YT videos:

~~https://invidious.fdn.fr/watch?v=lU4vv7qCQvg~~ (see update)

Exceptionally, if you opt to download it it merely opens a player to watch realtime. While other downloads from the same invidious instance have no issues. Why is this one getting different treatment?

update Apparently it’s an instance-specific problem with that particular video:

works → https://invidious.fdn.fr/watch?v=lU4vv7qCQvg

broken → https://iv.ggtyler.dev/watch?v=lU4vv7qCQvg

I’ve seen other instances where this particular video download is broken. AFAIK, invidious.fdn.fr is the only place where it works as expected.