Let's say I have a diverse herd of Elephants. I set up auto-slaughter like on the second screenshot. The oldest pregnant Elephant (12) is slaughtered instead of the overall oldest Elephant (13).
You have the "slaughter" checked on both Pregnant and Bonded. This means that any of the pregnants are a viable slaughter options as well as the non-pregnants. Game just picks any viable options that meets the adult age minimum requirement for slaughter. Since you're saying you want 32 total, it counts the male adults, female non-preg and the female preg and any that met the requirement over that total are candidates for slaughter. If you don't want your pregnants as option, don't have it checked, same with any that are bonded.
Instead, change the infinite sign on adult males or non-pregnant females to a set limit. This will cull the herd in the same way without the pregnants being options.
Can you read and analyze what 'the game' says in the lower right corner of the second screenshot?
Thanks for the bug report and we apologize for the inconvenience. It does look like the incorrect elephant was slaughtered, due to priorities. If you have a save of this happening so I can experiment with what is going on in your save specifically. that would be greatly appreciate. You can see how to get me your save in the "How to report a bug" stickied topic.
Sorry, I don't currently have a save file, or Rimworld installed, because when I do I can hardly stay away from playing. ))