TurboTax is working correctly. Due to the changes in the tax law for 2018, the standard deduction has almost doubled in 2018 (for years 2018-2025). For 2018 the standard deduction is $12,000 for single, $18,000 for head of household, and $24,000 for married filing jointly.
TurboTax automatically calculates your itemized deductions as you are entering them and comparing the total to your standard deduction. If your itemized deductions are higher than your standard deduction after entering all your itemized deductions, you will see a change in your tax.
Please see the TurboTax FAQ on how will tax reform affect my 2018 federal tax return.
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