A post description cannot contain a link or a footnote. The link I kind of understand, but a footnote seems like it should be possible.
This behavior also leads to a bug where a footnote will be inserted wherever your cursor was last before you clicked into the description. It's a bit difficult to explain, hopefully the video is more clear.
https://bsky.app/profile/pinkhatbeard.com/post/3mx7srtwnys2z