I was in my friend's realm and was going to make a zoo and wanted a few hostile mobs in my zoo. So I put name tags on the mobs and had them sit in separate boxes until the area they were going to was ready so I checked back on them and found that they were gone. So I decided to use the +

nbt tag and the /summon command+ so they wouldn't despawn. So I did. But then I came back to them all gone. The mobs that i had and  despawned were Chicken Jockeys, Spider Jockeys(the skeleton depawned), and Zoglins if that helps. I'm not sure if this is the same case in single player nor how many different hostile mobs have this issue.