Some used sponges are disappearing.

While clearing water from above a monument, I had two small 4x4 area sectioned off with sand walls above an ocean section.  I jumped down into the water and started placing sponges along the walls of the first blocked off area, working my way, clearing water, down to the ocean floor.  As I made it to the floor, As I returned to the surface level once the water was cleared, collecting the sponges, I noticed that at least 9-10 sponges were missing.  I removed the water in the second area, and this time I counted the sponges as I used them, clearing the water.  This time, as I made my way up, exactly 9 sponges were missing.  So placed sponges seem to be disappearing for some reason.