Perhaps not a bug but inconvenient behaviour.
If you change namespace of your XML instance then validator can't find
schema for this namespace. And in this situation validator behaviour
varies -- some will complain about missing declarations and some will
skip validation of element for which there is no corresponding schema
declaration.
IMHO, that's a sure bug.
We're asking an XSD validator, to validate XML instance document x with XSD document y, and tell us whether the result is valid or invalid. If the XSD validator reports a valid outcome, and if the validation outcome should have been invalid, then that's a bug.
