ISNULL in the SQL subquery does not return anything

Below is part of a SQL triggered that uses a subquery to return a value in the column TableString. The subquery will return the first two value and the "marker" for Fname, however nothing after that (not even the semicolon). The TableString column is an nvarchar that is set to 255 characters and captures all similar data during an insert or an update.

INSERT INTO TransactionLog (TransactionDate, Operator, TableName, Action
                           , TableString, UserId)
SELECT LastChangeDate
     , 'Op'
     , @tableName
     , @action
     , CAST('sNum:' + CAST(sNumber as nvarchar(10)) + ' entType:' + EntityType
            + ' Fname:' + ISNULL(FirstName, 'NULL')
            + ' Lname:' + ISNULL(LastName, 'NULL')
            + ' suff:' + ISNULL(NameSuffix, 'NULL')
            + ' corpName:' + ISNULL(CorporateName, 'NULL' )
            + ' ctrlId:' + ISNULL(CAST(ControlId as nvarchar(3)), 'NULL')
            AS nvarchar(30)) as TableString
     , LastChangeOperator
FROM deleted

Returned values in TableString:

sNum:1000024 entType:S Fname

---

This has nothing to do with ISNULL but rather, is the result of a truncation of data.

Pay attention to:

.. AS nvarchar(30)

That is, much more data is being omitted. (The output does not even contain "Fname:" in this case.)