I'm sending a POST to a server. I noticed, using fiddler, acrobat sends the request twice. Once without the authentication headers and one with the authentication headers. I have two servers, a test server, which sends back a realm with the 402, and a prod server that has no realm. Acrobat will send the request again with the proper auth headers for the test server (i think because there's a realm), but will not send another request to the prod server, I'm assuming because it lacks a realm.
I can't find any other reason for this except to assume acrobat wants to know what realm the auth will be used in. I tested this in POSTMAN, it works for both servers, but the authentication header is sent right away (no need for two calls).