When authorized fetch mode is enabled on mastodon
https://docs.joinmastodon.org/admin/config/#authorized_fetch, mastodon
will signatures for endpoints which are used for
activitypub or an authenticated user for internal endpoints.
Since custom emojis is internal, it cannot be accessed without a
logged-in user.
This improves the error to make it more clear.
Renato "Lond" Cerqueira