Autosave doesn't take custom Post Meta Data

[SteveAtty]

When Autosave fires the $_POST array only contains the basic information for the Post. Any custom post meta data is not transferred which means that unless a user then does a hard save before quitting the Edit Post screen any changes to custom meta are lost.

[nacin]

Autosaves aren't designed to be real saves -- it's a stopgap to ensure you don't lose data accidentally. This data means content, title, and such. Everything else indeed isn't saved by design. You need to do a hard save.

Additionally, most custom field actions get saved via AJAX from the custom fields box.