Hi,
I’ve just bought this plugin, and testing along with WP All Import, I saw that when a product’s variation has no price setted, it is not listed at all at the “Price by User Role: Per Product Settings” product metabox.
Debugging the problem, I fixed it with the woocommerce_hide_invisible_variations hook, at the line 240 of the includes/admin/class-alg-wc-price-by-user-role-settings-per-product.php file this way:
add_filter('woocommerce_hide_invisible_variations', '__return_false');
$available_variations = $_product->get_available_variations();
remove_filter('woocommerce_hide_invisible_variations', '__return_false');
Since, at my point of view, there is no reason to hide invisible variations at the edit product screen, I think this plugin should be updated with this fix.
I hope I have helped.