Named mob Despawning

First i named 2 patrol captain so if i want to have a raid ill just kill them but after 1 day they despawned. Second i have 2 villager and i turn them zombie villager so i can have a discount i search in wed that villager turn into zombie villager wont despawn i named them too,to be sure that they wont despawn but after i go to nether to get a nether wart to make a potion of weakness my zombie villager despawned so pls fix this bug. Thanks