I have two Strats, have played Fender guitars for over fifty years; after several problems with rust spots, mainly on the top three strings, I stopped buying these strings. Even the paper envelopes had rust marks from the strings.
Lured by the promise of new sealed packaging, supposed to cure this problem, I bought some more.
Two new packs already suffered from corrosion when opened, Thomann replaced the unopened packs, so two packs lost.
I just opened the first of the replacements to find the same problem. Awaiting Thomann's response.
No more Fender strings for me!
Shame, as I love the tone, the bullet ends never get stuck in the trem, and they keep their tone well.