When you have a sheet of ice with blocks beneath, it is possible that entities such as hostile mobs might spawn on this sheet of blocks, all together ignoring the ice. The intended behavior should be that the entities either do not spawn or spawn on the ice, not in it.
To replicate:
1. Make a sheet of blocks. 10 by 10 is adequant.
2. Coat the blocks with a one thick layer of ice. As long as the lighting is dark enough and game mode is easy or higher, mobs should start spawning in the ice.