skeleton spawner under sand spawns mobs that don't die or move

I created a custom world with the "beach" biome as the entire world biome. World Cheats where turned on. Dungeons where turned up. I started following a river from spawn and came across a skeleton spawner in the sand. Sand had fallen onto the spawner and there where skeleton heads sticking up out of the sand. I went into creative mode and started breaking the blocks the skeletons where buried in. The skeletons where not moving, they just stared ahead frozen. It was daylight and the skeletons where not burning up. The skeletons stayed frozen and kept spawning in the sand blocks even as I broke blocks around them.