Yes, these are plastic, but a soft material like cardboard can't work its way to the surface and the record isn't compressed, in order to cause the impurities to migrate. If something in the mixed PVC undergoes some kind of reaction and expands, I can understand it- maybe the impurities were able to do that. The cardboard I saw was from an LP pressed in the late-'70s and I blame the people who were working in material handling.
The plastic used for LPs is PVC with Carbon Black and it's hard enough that any impurities won't likely move. No limit to the time? That's absurd. Yes, the definition of 'plastic' applies, but these aren't made from softer plastic materials like Polyurethane, Polyethlyene, UMHW, CPVC, etc.
If you can show some of these, I would like to see them.