I had the same problem after updating to the latest woocommerce.
My temporary solution to this problem is from this
https://stackoverflow.com/questions/51310107/make-checkout-fields-required-in-woocommerce-checkout
It forces the checkout fields from optional to required. vice versa
Woocommerce checkout fields references
https://docs.woocommerce.com/document/tutorial-customising-checkout-fields-using-actions-and-filters/
Hope it helps
