Mastodon API: updating a media attachment's description or focus fails

[VyrCossont]

No matter what arguments are used to the update media attachment API method, the description and focus of the resulting attachment are always null and {"x": 0, "y": 0} respectively. The resulting post will not contain any alt text.

Images posted through the Pixelfed web API with alt text do include alt text when retrieved through the Mastodon API, so this is only an issue when posting.