Restore user in DataBase that in master have mapped the child database but in child database doesn't have the user created
Your login DEFCON\axadmin is sysadmin on the current server.

This means that it cannot be mapped to any database as DEFCON\axadmin user, it's already mapped and it's mapped as dbo in any database.

To see this just impersonate this login and ask for a user in that database:

use DAXTest;
execute as login = 'DEFCON\axadmin';
select user;
-- you'll see 'dbo' here
select select IS_SRVROLEMEMBER('sysadmin');
-- you'll get 1 here
revert;