Bees inside of bee hives / nests sometimes despawn when the world is reloaded

Bees despawn.  they are enclosed and so have not wandered off.  the bees were a mix of bees spawned on tree growth and bees that were bred from those.  They did not disappear when logging off/on, so I think it was due to distance of >128 blocks from player.  Bees are not in spawn chunks.

code analysis by Jaime Costas Insua here