Can't follow certain Mastodon accounts

Several Mastodon accounts give a "not found" on the well-known endpoint:

https://mostr.pub/.well-known/nostr.json?name=unix_discussions_at_mastodon.social

https://mostr.pub/.well-known/nostr.json?name=deutschewelle_at_squeet.me

https://mostr.pub/.well-known/nostr.json?name=tagesschau_at_dis-le.de

Is this a federation issue?