When an element wildcard with processContents="lax" is encountered, or when an element has a declared type of xs:anyType, any attributes on that element, or on a descendant element, whose name matches a global attribute declaration should be validated against that declaration. Saxon is not performing this validation.
(However, Saxon does validate attributes against a global attribute declaration if they match an attribute wildcard with processContents="lax").
This problem has been fixed in the development branch. No patch is being produced for the 9.0 branch because the risk of destabilising other paths outweighs the benefits.