For Cloudflare DNS servers the archive.md DNS server tells them the URL is not configured. It's configured to change according to which DNS server queries it by location/IP. It's not something Cloudflare can fix unilaterally. (or something like that)

edit: elsewhere linked is an explanation: https://news.ycombinator.com/item?id=19828702

A great example of why you as a techie should be using your own recursive resolver.