The account Steve does exist. It has the UUID 8667ba71b85a4004af54457a9734eed7.
This can be verified via https://sessionserver.mojang.com/session/minecraft/profile/8667ba71b85a4004af54457a9734eed7.

https://api.mojang.com/users/profiles/minecraft/Steve returns a 204 No Content instead of returning the UUID as usual.

Making a POST to https://api.mojang.com/profiles/minecraft with ["Steve"] returns an empty array instead of returning the UUID as usual.

I think this issue may be related to user 0621d1ddc64d4d8285be91995a753c53 who has renamed their account from steve (with a lower case s) to Volcano_man on 17 Feb 2015 04:31:28 UTC, see https://api.mojang.com/user/profiles/0621d1ddc64d4d8285be91995a753c53/names
I think this issue may be related to that user.

does work: https://api.mojang.com/users/profiles/minecraft/Steve?at=1424147488
does not work: https://api.mojang.com/users/profiles/minecraft/Steve?at=1424147489