Coins in Steve Ivy flips

I picked up some coins that were in "Steve Ivy" flips many years ago. The 1929-s coins were all graded "gem BU 65" the two 1912d Lincoln cents 60+ red and brown, the 1909 was graded CH BU 60+. The 1929-s coins look as if they have environmental damage, which the 1912d cents look no problem. Why would some coins in the pvc flips develop issues like this while others not?
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Comments
Because PVC flips don't all degrade at the same rate. Some might be fine for decades. Others will degrade in a few years
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From a couple 1929 Lincoln cents that I have seen, I get the sense that the alloy used that year was slightly different than most other years. @robec can probably add to this.
Young Numismatist • My Toned Coins
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Did you get all those Steve Ivy flips in one lot?
A shop had the group in their old inventory, I picked them up yesterday. Once cents react like this I assume there is no way to conserve them.
Nice
looks like some cool stuff