Mobs when placed by a Dispenser (facing upwards) are summoned with their head in the block above the spawner (such that they can suffocate in the block above when solid, or spiders are summoned within the dispenser block itself)

A suggestion to fix would be to make it so mobs have their heads spawned 3 blocks above the dispenser (i.e. feet on block immediately above dispenser). This would allow dispensers facing upwards to be placed underneath flooring for dungeons and other fun stuff.