Name-tagged mobs despawning

Returned from mining a  chunk near my base that I have been to many times before to find several name-tagged villagers and my name-tagged horse have despawned. Really annoying as villagers were at max level and I had name-tagged them thinking this would prevent despawning. Also my horse had full armour and saddle. None of my other name-tagged animals were affected. 

why is this happening? Title page says I’m on v1.16.50 but your drop down box doesn’t recognise this.