Hi,
It's probably because they entered their billing address before you activated the "required" option in the custom field.
However, even this should be covered by the improvement
There is now a check to prevent someone from validating a checkout with an old address missing information in required custom fields that were not required when the user first entered his address.
we've added in HikaShop 4.3.0 one year ago:
www.hikashop.com/support/documentation/5...ashop-changelog.html
So I'm surprised you're saying you have this issue with the 4.4.3.
The best would be to run a mass action to delete the addresses without the VAT number field filled in. That way, these users would have to reenter their address with the VAT number not being filled in and they wouldn't be able to enter it without the VAT number.