Here is what I have determined are the problems with the google feed plugin-
1. Sometimes, feed for store1 does not include 'brand' or 'product_type' attributes at all. It must be verified prior to uploading to Google merchant center or all your listings will be removed!
No particular order of steps can be taken to reproduce the error. It seems entirely random. If you have 2 or more stores, and click the generate feed button, just be sure to look at the feed (after refreshing it) and ensure the brand attribute is included on each product before uploading it or allowing google to fetch it.
2. The generated feed for store2 does not include 'product_type' attribute unless the product has a category that is limited to store1.
3. When generating the feed for store1, the feed for store2 is also generated at the same time. This is only a problem because of problem #2 above. Because the product_type attribute is not being included in feed for store2, I manually add product_type to each item. But when I generate new feed for store1, my store2 feed is also overwritten..and I lose all the product_types