It has to do with the player moving a long distance from the mob and the mechanics of chunk loading and unloading. Apparently if a chunk gets unloaded, some mobs (dolphins etc...) within disappear.
Hi, Jake! Haha, yeah, I had no idea what the "Environment" field meant. But this issue of dolphin suffocation on a leash, may be partly responsible for numerous "dolphin despawning" bug reports.
It has to do with the player moving a long distance from the mob and the mechanics of chunk loading and unloading. Apparently if a chunk gets unloaded, some mobs (dolphins etc...) within disappear.