I sure this a very small number, but i also think its a worry that it could happen at all. Never had a set of string with rust on them before, let alone brand new ones.
I’d also like to think that although we are a large community there must be 1000s not on web forums, sp who knows how widespread it is. It does seem that those who got a bad string/set got them replaced with good ones, and if you do get a bad one just send the set back. Nothing to lose really, other than maybe time, and not liking them.