$0 is usually correct. Automated tax calculates based on nexus. No nexus in the customer’s state = zero tax. Not a bug.
If you expect tax and it’s $0: Add a Standard Tax Rate for your nexus state with a wildcard (*) that automated tax can override. See section 2.3 for the setup.
If tax was working and suddenly shows $0: Check the automated tax service is active. Check if a plugin update changed settings. Check the product’s tax class. Verify the test address is in a nexus state — people often test outside their nexus without realizing it.