have you checked the state field for the address?
if i go to what i think is the problem site, and i go to create an account, the config looks problematic as there are 2 fields for the state. see:
you have a state pulldown as well as a state field for the user to enter (this does not occur with an account and editing an existing address).
its a pure guess, but i'm guessing the tax problem may be related to the config of the state field.
best.
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