I've just checked my coins immediately after reading this post (didn't take long because I have only a few).
Maple Leaf 2013 - I was aware one of the coins had a milk spot that appeared a few weeks after I had gotten them (same year as series). But it had not deteriorated since then. The others, including Australian Kangaroo 2016, US Walking Liberty 2016 etc. are free from milk spots too, thankfully.
I'm surprised by the extent of your coins here. Might weather be a factor? I'm in Malaysia where it's hot and humid most of the time.
They aren't my coins. I was being lazy and just showing the worst examples I could find.