I and others playing on my realm have had an issue with mobs despawning.  This has affected axolotls, goats, and frogs.  Having bred the mob or named it with a name tag has not made any impact on the issue.  The mobs have not despawned when they have been attached to leashes; however, the leashes break randomly, with no obstruction or other reason for it to happen.  Once the leashes break, the mob attached to the leash has a high chance of despawning.  This has been a continuing issue over multiple updates.