(I've tried for about 10 minutes trying to see if this issue already exists, doesn't seem like it does. Might be related to this: https://bugs.mojang.com/browse/MC-150170?focusedCommentId=555215&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-555215)

When putting mobs inside a Minecart and unloading the chunk, the mobs will have disappeared when returning back to the chunk.

To replicate this, trap some mobs inside a minecart. Fly far away from the chunk so that it unloaded. Wait a minute or two and return. The mobs would have disappeared. Can't tell if they despawned or died. 

Here's a Github issue report of a user talking about it and me explaining my test: https://github.com/Kyrobi/Cynagen/issues/51