Minecraft API return <Respone 204> even if some usernames exist

If you try to get the api page of 02_ (https://api.mojang.com/users/profiles/minecraft/02_) it returns 204, as if the username is not taken, but if you try and rename yourself in this username it says username ist already taken. This happens with many names.
NOTE: This only sometimes happens. Other times everything works fine.

Examples:
02_
03D
03x