Sql-server – Trouble with fixing orphan users

I am creating a script where I have to fix orphan user (app service account) when db gets refreshed from Devl –> Test –> Prod. Since my company sells this software, it is essentially huge number of servers and I alone cannot manage it. So, I decided to create a script and give just that permission to application support people so that they can run that script which should fix orphan user.

This is the script

CREATE PROCEDURE [dbo].[usp_FixPermissionsAfterDBRefresh]
AS
BEGIN

    DECLARE @dbname VARCHAR(50)
    DECLARE @sqlstring NVARCHAR(100)

    DECLARE db_cursor CURSOR FOR
    SELECT name
    FROM sys.databases
    WHERE name NOT in ('master','msdb','model','tempdb')


    OPEN db_cursor
    FETCH NEXT from db_cursor into @dbname

    WHILE @@FETCH_STATUS = 0
    BEGIN

            SET @sqlstring = N'USE ' + @dbname
            EXECUTE sp_executesql @sqlstring
            print @sqlstring
            --EXEC sp_change_users_login 'Auto_Fix', 'SomeUser'
            IF EXISTS (SELECT dp.name  As Orphan_Users
                        FROM sys.database_principals dp
                        left join sys.server_principals sp
                        ON dp.sid=sp.sid 
                        WHERE sp.name IS NULL 
                        AND dp.type='S' AND 
                        dp.name NOT IN ('guest','INFORMATION_SCHEMA','sys'))
            BEGIN
            ALTER USER SomeUser WITH LOGIN = SomeUser
            ALTER ROLE [db_backupoperator] ADD MEMBER [SomeUser]
            ALTER ROLE [db_owner] ADD MEMBER [SomeUser]
            END

        FETCH NEXT FROM db_cursor INTO @dbname
    END

    CLOSE db_cursor
    DEALLOCATE db_cursor
END
GO

If I run this script from any database except the one that got restored, the script exits successfully but won't fix orphan user and grant permissions. However, if I run this script from the newly refreshed DB then the script works just fine and fixes orphan user and grants necessary permissions. I have searched online but couldn't really get this script working. I will be setting this up as an agent job and grant permissions to application support who will be performing db refresh.

Please advise on what I am doing wrong.