I haven't slabbed any.
A lot of the cracking is from the plastic losing some plasticizer over time and shrinking while the paper doesn't shrink. Humidity can also make the paper expand, and if things are restrained in a holder I think cracking is probably almost inevitable.
Some of my factory sets curled enough in the original package that the panels separated into individual cards. None cracked, at least not by the last time I checked. Most of my loose cards have curled some. Oddly the earlier proofs are mostly flat and the plastic is still more flexible. I believe it's a different type of plastic.
Plastics are a "new" enough material and there are so many specific formulations both in production and processing that even professional conservators in museums are often puzzled about how to go about preventing it from damaging itself.
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