Thank you.

We have found the root of this problem. WooCommerce system doesn’t provide frontend with all variations if their number is bigger than 40. And because of that images for some variations are not available in the template file. You can fix it by adding the following line to the file basel/woocommerce/single-products/add-to-cart/variable-swatches.php between lines #38 and #39

$available_variations = $product->get_available_variations();

But it may be bad for the single product page performance if you will not use any kind of cache.

Regards