Hi,
The VAT is present before login/registration on your website:
monosnap.com/file/9L9qRyTnbGJuSRMone8GwvPh6DzH46
So the product is setup correctly, and the HikaShop configuration is setup correctly, and the tax rules are setup correctly for when the user is not logged in and it uses the main tax zone.
So it must be something between the user address or the user itself, like his user group leading to the tax rules filtered out.
I can't say anything more precise without more information on the problem.