When I add a property tax deduction my federal taxes do not change, no matter how large.
I am married filing separately and my spouse and I are itemizing. In past years, changing this value always reduces federal tax. This year, no matter what I enter (even very large numbers) the federal tax stays the same.
I checked Federal Information Worksheet Part IV and verified it says that I am filing married separately and my spouse is itemizing.
If I change status to married filing jointly, increasing property tax deduction decreases Federal tax.
Also, there are phase-out rules for certain itemized deductions (including property taxes) -