I was told by someone that the /authenticate endpoint changed, and in the process of guessing at the meaning of the new fields, noticed that the migrated field is always false for both the selectedProfile/availableProfiles and user fields. The system knows that the account has been migrated, since the migratedFrom and migratedAt fields of user (when requestUser is true) have correct-looking values; it's only migrated that is incorrect.

To reproduce, run something like the following curl command (with the proper username and password for a migrated account):

curl --data '{ "agent": { "name": "Minecraft", "version": 1 }, "username": "user@email.example", "password": "Hunter2", "clientToken": "curl-test", "requestUser": true }' -H "Content-Type: application/json" https://authserver.mojang.com/authenticate

and then observe that migrated is set to false despite the account having been migrated.