It looks like Dawid created profiles/default/linux/amd64/package.use.mask to resolve bug #238487 via masking a USE flag. However, this is the incorrect location for such a mask. Instead the mask should be moved to arch/amd64/package.use.mask since everything below default/linux should only have parent files or profile-specific changes. Thanks,
Thanks Chris for spoting this oversight. I've just fixed this in cvs.