Yes. They are vaporizing. Modern frost-free fridges have a brief warm cycle periodically to melt the frost (ice) that develops in the freezer from moisture in the air. The air in the freezer is colder and hence dryer than normal air because cold air holds less moisture than warm air. So what happens is the ice eventually evaporates. This is a process called "freeze drying" or lyophilization. You can buy equipment that is optimized to freeze dry stuff even faster and better than your fridge. A lot of light-weight back-packing food is made by freeze drying.