I added a required customizable option (not sure about the name in English, in French, it's "Option personalisable").
On the front, there is no problem until you try to add the product to the cart. At this moment it's printing you this message :
"The product's required option(s) weren't entered. Make sure the options are entered and try again."
I tried to get where the problem was happening in the code, and I found the place where the error was triggered : vendor/magento/module-catalog/Model/Product/Option/Type/DefaultType.php l216. When I tried to print get type($value), it gave me a "null".
I tried to go further in the core but cannot find where the bug is happening.