While playing in a server we've started noticing Sniffers we found would randomly disappear, that issue didn't seem to happen with any other passive mob we had around. We put nametags on the sniffers to see if it prevented their despawn or to see if they were dying somehow. While I was online another sniffer had disappeared again, checking the log there was no mention of their death.
This issue seemed to happened while the chunks were unloaded since all the players in the server were far away from them when it happened.