Facebook Catalog wrong Availability

We set up Facebook Catalog and Availability field is not transforming correctly. In your blog you write:

The plugin pre-mapped Facebook’s availability field to the stock status of your products. Allowed values for Google are “in stock”, “out of stock” and “preorder”. The plugin will automatically determine if your product is in stock or out of stock so leave this pre-filled mapping in place.

Instead the Availability field results with “on backorder” text.