The WeldContainer.instance() method returns an instance with the @Default qualifier (e.g.):
@Inject @Default private Instance<FooInterface> instances;
So when you do select() you get an @Default @FooA Instance, and no beans have both qualifiers.
WeldContainer is not specified by the spec, but this behaviour is unlikely to be changed as existing programs will depend on it.
For an example of how to get an instance programatically, see: