Parameter validation works randomly on Weblogic EJB

Parameter validation works randomly (this is, sometimes validates and sometimes it doesn't with same data) when validating ejb parameters in Weblogic (no problems on JBoss). Weblogic enhance both, ejb interface and ejb implementation.

Test case: build and deploy the ear on Weblogic (12.1.3) developer edition. The request http://host:port/bug-test-case-war/test?value=invalid-parameter sometimes validates and sometimes do not.