I own a Minecraft Realm and have recently built an iron farm that utilizes a zombie in a minecart going around a track. I got the zombie into a Minecart with full armor and an item and the iron farm began to work. After a couple of hours the zombie disappeared, presumably despawning. I went into creative mode and spawned a new zombie giving it the same items and putting it back in the minecart which reproduced the issue. I then tried additionally naming the zombie even though it could already not be able to despawn when holding an item. Then after a few hours it once again disappeared.