Passive mobs dying in enclosed area

So I tried making a standard cow farm, made a pen in the ground 3 blocks deep, all surrounded by dirt and grass blocks, 8x8 blocks wide. I had 2 cow originally and eventually made it so I had 12 cows. After a little while, I know I interacted with all the cows, but when I was in the pen, one of the cows died randomly, I saw it happen, it dropped items, nothing else was in the pen, only the cows and only 12 at max were in there so I just thought nothing of it since it was just one cow but then I leave my farm, it is like 3 chunks if not 4 or 5 from my base, closer to spawn, and half of my cows are gone. I co go old only assume they either died or but I can't make a standard cow farm in this version. I didn't update in the process, I already had the game updated so it couldn't have been that. I did close the world several times before checking on them but I don't see how they would just die or despawn. There was no reason for it. I wasn't playing with anybody else either.

Minecraft version: beta 1.6.0.6

Phone: Samsung Galaxy Note 8

Android version: 8.0.0