Upsell Rule Showing Incorrect Category

We have multiple rules setup for categories on the website, however have noticed when the category has less than a certain amount of products in it, the product recommendations seem to show a random seemingly default category?

A product in question is this: https://www.fundamental-changes.com/book/the-complete-dadgad-guitar-method/ it is only on one category (https://www.fundamental-changes.com/product-category/guitar/dadgad/) and we have a rule set for this called ‘DADGAD Category Product Recommendation’ to ‘Include Cart Items By Categories’ with only that category included. Yet if you add this to cart and go to checkout, you’ll see the product recommendations are for Piano books, which is totally wrong?