F.Y.I.

I did get a confirmation from Microsoft that this is a bug generated by SP4.

This problem is contributed by the data type inconsistency in the system tables. SID column in sysprocesses is a binary(86), while defined as a varbinary(85) in syslogins.

The work around is to cast the SID to a valid fix length of binary(28) in order for suser_sname() to work.

Microsoft has no known fix plan for the time being. They note Yukon isn't affected by this bug.