I've looked through other tax calculation issues on Stack Exchange and believe that my issue may be unique. I'm running Magento 184.108.40.206 with TaxJar 2.3.3. I'm also using Inchoo_PHP7 so Magento runs on PHP 7.0.29, in case that matters.
So, my problem: When a customer is located in the state of Texas where we have a Nexus, the tax rate is being calculated incorrectly. Instead of the correct rate of 8.25%, the tax rate is being calculated at 8%.
I use TaxJar to maintain the correct tax rates for Texas and Louisiana, since we have a nexus in each state. Those tax rates all seem to be correct. Texas, in particular, has a single state-wide rate of 8.25%.
Under System -> Sales -> Tax, I've got it configured to have no default tax destination calculation. So the user isn't shown taxes until they've entered their destination state, either through the shipping estimator or when entering their shipping address.
If I change the Default State to Texas, then I initially get the correct calculation (8.25%). But the moment the customer does a shipping estimate or enters their shipping address (in Texas), the calculation switches to the incorrect rate (8%).
How can that be? How could setting the Default State to Texas differ from the customer entering a Texas shipping address when calculating the tax rate?
I have pored over the tax tables, System settings, etc and found no tax rate setup for Texas that's 8%. Nor have I found a default tax rate that's 8%. The only other broader tax rate entry would be for Wholesale Customers, which is 0%. So that wouldn't be the issue.