When navigating to https://sessionserver.mojang.com/session/minecraft/profile/lookup/bulk/byname it gives this error:

This error is present regardless or not if a username is given after the 'byname' section, or if the 'byname' section is replaced with a UUID.

I'm fully accepting that this may be my usage of the API that is wrong in which case I'm happy for someone to point that out so I know how to properly do it!