Hi,
1. Not at all, this was a known bug we fixed, and it seems it applied to your case as well. It explains why only some of the products were affected.
2. The default solrting is relevance by default. Of course, if you want to change that, it is possible. When usin WooCommerce, the popularity is stored in a custom field (total_sales), so you can set it up like this: https://i.imgur.com/P3huWU8.png
