IS_SRVROLEMEMBER returning NULL for valid non windows user
I think you're confusing logins and users


logins exist in sys.server_principals at the Instance level
users exists in sys.database_principals in each database


Your EXECUTE AS is executed based in info in sys.server_principals but requires an entry in sys.database_principals.

Here, I have a login gbntest that maps to user gbntestU in my GBN database.
My stored procedure works OK

USE master
GO
CREATE LOGIN gbntest WITH PASSWORD = 'pwd'
GO
USE GBN
GO
CREATE USER gbnTestu FROM LOGIN gbntest;
GO

--0, OK
DECLARE @loginuser varchar(100) = 'gbntest';
SELECT IS_SRVROLEMEMBER('sysadmin'), IS_SRVROLEMEMBER('sysadmin', 'JoeBlow'), IS_SRVROLEMEMBER('sysadmin', @loginuser);
GO
--NULL, expected
DECLARE @loginuser varchar(100) = 'gbntestu';
SELECT IS_SRVROLEMEMBER('sysadmin'), IS_SRVROLEMEMBER('sysadmin', 'JoeBlow'), IS_SRVROLEMEMBER('sysadmin', @loginuser);
GO

CREATE PROC dbo.uspGBNTest @loginuser varchar(100)
AS
SELECT IS_SRVROLEMEMBER('sysadmin'), IS_SRVROLEMEMBER('sysadmin', 'JoeBlow'), IS_SRVROLEMEMBER('sysadmin', @loginuser);
GO
EXEC dbo.uspGBNTest 'gbntest' -- get 0, OK
EXEC dbo.uspGBNTest 'gbntestu'  --get NULL, expected
GO


So, what do these give in the stored proc?

SELECT
  SUSER_ID(@loginUser), -- sys.server_principals
  DATABASE_PRINCIPAL_ID(@loginUser) -- sys.database_principals (for completeness)


This will tell you if


@loginuser is set correctly For example, TheUser is sent when it should be DOMAIN\TheUser
the expected principal (@loginuser) exists at both the Instance and database level for IS_SRVROLEMEMBER to work