With the latest update, Backpacks are no longer wiped on map wipe. I currently have the following setting in my config file:

The most common reason for this issue (for any plugin) is if you start the server (and generate the new save file) while the plugin is unloaded. A common reason for that is that Oxide was not installed yet.

Most likely, you are also seeing this issue for other plugins at the same time, if you have other plugins that wipe data on map wipe, but you might not have noticed yet.

Got it - thanks!