I can't see how the choice of adhesive (rubber cement vs. Slime vs. Rema etc.) could make any difference, assuming the air is leaking through the cracked patch. The glue is just holding the patch to the tube, and unless air is leaking out around the edge of the patch, the glue is working.

Since we're all just speculating here, I'd guess there was some incident during the hanging storage time. Heavy weight folding the patch over, and the inflexible patch cracking perhaps. Odd that it affected just the patch, unless the patch had been stored next to a blower motor, perhaps in the warehouse or store from which it was purchased. That kind of ozone exposure might harden the rubber in the patch, while the tube to which the patch was attached was safe and sound inside a bicycle tire.