Ghasts keep despawning with leash on

So I found a ghastling one time and let it grow but when I had it on a leash on a pole it despawned after some time! I’m not sure why. I’ve tried another time keeping it in a boxed place but it still despawned and no death message came as well. This is getting very frustrating and I hope this bug will be able to be fixed. Or if there’s a way for me to make them stop despawning I’d like to be told how!