SQL Server 2005 / Securityadmin Role / SSMS

Experiencing a little weirdness with SQL Server Management Studio...

Have created a login, that has fixed server role "securityadmin".
The login can create/drop logins, add/remove them as users in
databases, etc.

The login can change another login's password, using SQL
from a query window (e.g. alter login bob with password='abc')

However, the login *cannot* change another login's password,
using SSMS GUI (e.g. "Security", "Logins", double-click "Bob",
change password, say "OK"). The error text is:

Change password failed for Login 'Bob'. (Microsoft.SqlServer.Smo)

Additional information:
An exception occurred while executing a Transact-SQL statement or
batch.
(Microsoft.SqlServer.ConnectionInfo)
Cannot alter the login 'Bob', because it does not exist or you do
not have permission.
(Microsoft SQL Server, Error: 15151)

Has anybody else experienced this?