I have a personal server running on my Ubuntu and there is a bug with fish, dolphins, and squids that are spawned from eggs. They disappear/despawn while I'm on the server, and when I spawn them before save-all and quitting, they are gone upon my return. 

I thought the issue may have been RAM, so I increased the allotted space from 1GB to 2GB but this hasn't resolved the issue either.

Break down

• Fish (all fish), Dolphins and Squids spawned from Eggs end up despawning while other mobs spawned from eggs remain and are unaffected
• Wild caught fish (all fish) in a bucket then placed elsewhere remain and are unaffected