You apparently paid Alternative Minimum Tax in prior years, and depending on the circumstances, a credit for such tax can be carried over to the current year. This is reported on form 8801.
I suggest you go back to Itemized Deductions, and look at form 8801 to see what TurboTax did (if you can see your output forms).
It does not surprise me that you can see such large changes in your refund when dealing with the Alt Min Tax credit.
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