Unexpected Principal (Security Identity) Propagation Switch

Problem Definition:

I have a web application utilizing JAAS (form based authentication, DatabaseServerLoginModule), Struts, Session Beans and Entity Beans. This configuration is working successfully, but I have noticed an unexpected switch in the Principal that is associated with the EJB invocation layer when more than one call to a session bean is made from the web tier (Struts Action) within one request.

Permissions & Security Identity

Web Tier - User Credentials

• id=joe
• password=pw
• role=administratorRole

Session Bean (fooSessionBean)

• Permissions: administratorRole, internalRole
• Security Identity (run-as): internalRole

Entity Bean (fooEntityBean)

• Permission: internalRole
• Security Identity (run-as): internalRole

Note: fooSessionBean.bar() calls fooEntityBean.bar()

Scenario

1) User logs in via form authentication (j_security_check)

2) User clicks on link that invokes an action that results in 2 calls to fooSessionBean.bar().

2a) First call to fooSessionBean.bar() is successful.

2b) Second call to fooSessionBean.bar() fails.

Code:
javax.security.auth.login.FailedLoginException: No matching username found in Principals

I set a breakpoint in the JaasSecurityManager.isValid(..) (line 251) and noticed that this method is called on the 'second' invocation of the fooSessionBean.bar() where the
Code:
principal = [roles=[internalRole],principal=anonymous]
credential = null

It appears as if the security-identity (run-as) defined for fooSessionBean is replacing the original principal credentials of the user that logged in during the first call to fooSessionBean.bar().