Trying to add the GNOME releases schedule as an HTTP or webcal:// online calendar doesn't work (400 HTTP error)

Trying to add https://www.gnome.org/start/unstable/schedule.ics or webcal://www.gnome.org/start/unstable/schedule.ics as a calendar in GNOME Calendar 43.1 does not work, even though the UI indicates it should. The GtkEntry field is colored orange to indicate that a problem occured:

Running with --debug, this is the only line that occurs when pasting the URL in the entry in that dialog:

GcalNewCalendarPage: WARNING: Error finding sources: Failed to get properties: HTTP error code 400 (Bad Request)

Evolution 3.46.x has no problem adding this same calendar source.