So you are saying that your "free" proof set was probably picked over and that was the remnants of probably other impaired proof set coins.
If the Dansco has cardboard in it, that could be the start of your problem. In 60+ years of collecting, I have never stored Proof coins in those types of albums. I have used Capital Plastics, slabs or the original mint package. This Is why I am not a fan of red copper. Sometimes it seems to “go bad” out of spite. I know that there is really nothing else to collect for the modern coins of say the last 50 or 60 years. I don’t like messing with it, especially the Proof pieces. Preserving them with the red color is like trying to save a snowball for the 4th of July.
You are correct of course, it's the cardboard. But when the holes are there I feel the need to keep the sets complete this way. I do however like the toning of my silver and nickel coins, none of which has gone over the top.
I had a red 1903 indian head cent in my 7070. Looked at it 3 months later and it was beginning to turn brown especially on the reverse. Took it out and I've kept it in an airtite ever since. For what I know all copper will turn brown whether it takes a month or a hundred years.
This is precisely why I hate owning “red” coins. I prefer nicely-toned brown coins. Brown coins are also much cheaper