Why Doesn't my Authorization Header need "Bearer"?

I am currently working with a group of applications that are running on two separate, but equivalent, environments (Referred to as ENV1 and ENV2). I have been using OAuth 2.0 for Authorization and when I receive a response after requesting an access token from the OAuth service (I am making my requests through Postman) I get a response that looks like this from ENV1 and ENV2:

As far as I am aware, I believe that this "token_type": "Bearer" means that when I send in the access_token to my application I need to do so like this:

By sending in the token through the Authorization header, prefixed with "Bearer". This approach works fine on ENV1 but on ENV2 the request fails unless I send in the token alone with no "Bearer" prefix:

If I send in the Authorization header with the "Bearer" prefix, I get a 401 Unauthorized error as the response. This is the help tip that Postman provides(Emphasis mine):

Similar to 403 Forbidden, but specifically for use when authentication is possible but has failed or not yet been provided. The response must include a WWW-Authenticate header field containing a challenge applicable to the requested resource.

The problem here is that there IS a WWW-Authenticate header field, and it contains "Bearer", which I would assume is a "challenge applicable to the requested resource" seeing as the token response contained "token_type": "Bearer":

Questions:

• Why would this be different between environments?
• How is this even possible? The documentation I have found on OAuth 2.0 shows that the "Bearer" prefix is needed for requests like the ones I am trying to make. (for example, in section 2.1 of the documentation here)

From your description, it seems that the environments are not actually identical. E.g. maybe ENV2 is behind a gateway that adds the Bearer prefix to the header. Or the API on ENV2 (or the gateway) is configured to read the header without the prefix.

When the OAuth Server returns the access token, it gives you the type - a bearer token. That type means, that the token is just this - a bearer token - as opposed to a Proof-of-Possession token. When you send a bearer token to an API, you don't have to provide any additional information that would prove that you are the owner of the token. (you can compare bearer with the DPoP standard)

The Bearer Token Usage standard does require you to use the prefix Bearer in the authorization header (as you pointed out), but it doesn't mean that all the APIs and gateways implement that standard correctly, or that they use that standard at all.

To summarize:

• it's up to the gateway/API to decide in what format they want the Authorization header, and that has nothing to do with the type of the token (a bearer token). It's nice when they use standards, but they don't have to.
• In your setup, there must be some kind of difference between the environments if the same request is treated differently between them. If you own the environments you should investigate what is configured differently. If you don't own them, you should contact the owner's support to solve the issue.