I made a mob grinder/dropper based of this tutorial here:
https://www.youtube.com/watch?v=hQPeZlWG-kI

I have made this Mob Grinder/Dropper many times and multiple different versions. It would seem that it doesn't want to spawn anything on the windows 10 Edition of Minecraft. I went to see why and it looks like light is in there, but there is no light source and it doesn't even spawn anything at night.

So I think the mob spawning is broken or something.