Mobs despawn even with nametag

i made a iron farm with 20 villagers and with a nametag incase they despawn. But after a while i checked on the villagers and their left with just 9 of them left, if theres even a zombie it wont just leave 9 alive right?Ok maybe this is not enough though i have more. one time i named a zombified villager and put a nametag called 'dinnerbone' and their suppose to upside down them ,well it did but again after a while, and in that time i was mostly next to my base(which the zombified villager with the name tagged'dinnerbone' with it . Despawned... It doesnt make sence for a mob with nametag despawn .this bug has been in a few versions but i think mojang really has to fix this bug. Thanks