Yes, that is rather unusual. I would suggest there was something other than the GH involved. My tap water is 7 ppm (according to the water authority) which is less than half of 1 dGH, and I do nothing in most of my tanks to raise this (I have mainly wild caught Amazonian and SE Asian fish) but the MLS thrive. The only tanks in which they are not numerous are the two with loaches, and though the loaches have difficulty with MLS due to their "trap door" they do seem to get some. I once had a dwarf puffer for several months in my 10g, and he kept them down; I thought they were gone, but after the puffer was removed, over a few weeks the MLS became more numerous, so obviously some escaped predation in the sand.
MLS will survive freezing, and near-dehydration (they can live for months in damp gravel without actual water). Copper is said to be a problem for invertebrates, and this can occur from plumbing as well as medications if any were used. The level of copper in liquid fertilizers is not sufficient to kill snails unless way overdosed.
Byron.