Description Edited for Clarity...

When you have an untamed cat or ocelot on a lead and you attach it to a fence post, the cat will de-spawn after a short period of time if you move too far away from it. This has been tested over multiple versions of Vanilla Minecraft and is confirmed in each of those versions.

How to reproduce bug:
1. Spawn a wild cat with either commands or an egg.
2. Attach a lead to the cat.
3. Attach the other end of the lead to a fence post.
4. Move about 100 blocks away and watch the cat through a spyglass.
5. After 15-120 seconds, the cat will de-spawn and leave the lead on the ground.

Behavior expected:
Other mobs such as wolves, foxes and trader llamas do not de-spawn when attached to a fence post with a lead. It's expected that cats and ocelots should not de-spawn when attached to a fence post with a lead.