Replaced pool liner for 40'x20' inground Lazy-L back in 2015. Last Oct I lowered the water to about 10" below the returns instead of the normal 2"-3". Everything fine up until 25-Dec when i looked out and saw that the water had dropped sharply - down to maybe 2" of water on the shallow end. The night before it was cold enough for ice to form - as you can see in the photo there's still ice. I suspect the liner was pierced by ice but what I don't get is why wouldn't the damage be at the level where ice would have initially formed - at 10" below the returns. Why would the water have dropped that low?