Select function values not working on front end

Hi,

I have just installed this plugin and added the product field using the select function and key in numbers from 3 to 9 as the common ring size (online ring business) however after insertion and update, though the select drop down box was displayed however the values were empty inside and default value “6” cannot be shown. Appreciate if you can advise.

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https://www.ads-software.com/plugins/woocommerce-custom-product-data-fields/