Mobs spawning within areas that have been lit up

I was playing in my survival world when i realised i needed to get another repeater for my redstone contraption. I was gone from the area where the mobs spawned for approximately 5 minutes. When i retured 4 zombies had spawened. From the video you can clearly tell the area has been lit up by several torches. I have been noticing that happening alot in some of my other builds in the same world. However capturing evidence of the bug previous times has been pretty difficult as the mobs keep attacking me. Most of the time the mobs spawn in areas i have been in recently. Eg: after using my furnaces in my blacksmith hut at night and the going to sleep to skip to day, I would come back to see if my items have smelted and would find a group of zombies. I close my door after leaving my blacksmith hut so the zombies can't of walked in.