All mobs even when named with a nametag eventually despawn. I have experienced this issue with Turtles, Fish, Villagers, and Minecarts. This is a serious issue as nametags are supposed to prevent a entity to despawn but this bug causes them to despawn regardless. When I put a  nametag on a mob it stays in the world for multiple hours of playtime but eventually disappear one by one over a timespan of hours. Even villagers i have traded with before eventually despawn even though I have traded with them and name tagged them to prevent them from despawning they despawn anyway.