This can be resolved by changing the type of option name that these attributes are by use of the option names manager. Change them to a read only type. More can be seen or read here: https://docs.zen-cart.com/user/admin...an-option-name
I only have Read Only option type for the error causing products, have not mixed with any dropdown. I have added a multiple (about 6 or more) lines of Read Only values for each product. This causes equivalent lines of the error.