A witch spawned in my box right after I made it there were no entrances and there was light

So basically I boxed myself in because I had really good loot and I was about to die then I got killed by a witch and this shouldn't be possible because it spawned inside the my box and I just made the box it took like 5 seconds for a mob to spawn Im pretty sure that's not normal and there was light in the box so they shouldn't spawn. closed out the game after the death because I was annoyed that I died and lost all my stuff so I just quit the world and this caused me to forget to take a video of what happend