It depends on if the damage happened after the snail died or was the cause of the snail dying..... I leave my shells in the tanks so the calcium can be reasorbed into the water.
Ya, I've seen similar erosion of ramshorn and even MTS shells. Lower pH will cause the shells to deteriorate more quickly. In tanks with crushed coral to buffer the water chemistry shells will remain intact.
I also leave shells in my tank. I must say that the damage to the shell could very well have happened posthumously and that simply having CC as a substrate won't keep shells from deteriorating.