Most products for us show size options as greyed out when they're set to disabled. We have one product in particular that hides those size options entirely instead of just greying them out. The store setting for 'Display Out of Stock Products' is set to yes. Is there any reason this would still be hiding those production options?
If its showing on all the products and only one product have issue - then i think something is missing on this product only !
do one thing kindly compare both the products - product attribute - then check !
because if Display Out of Stock Products is set to yes - then it will displayed as it is.
Also try to clear the cache - by running the command php bin/magento cache:clean and php bin/magento cache:flush
then check it in private window , i think it will resolved your issue !
Can you tell me what you mean when you say compare the 2 product attributes? I just need more information to know what to do there.
It means , open the product edit page for both the product - one which size option is able to showcase on frontend part and open in a new tab second product which size option is not showing on frontend - then compare all the product attributes over there - something is missing on product configuration for that specific product.