The code Z entry in box 20 is for the qualified business income deduction (QBI). It should be the same sign as the number to which it pertains, which is the income reported in box 1 to 3 on your form 1065 k-1 schedule.
Code "N" would be a positive number, as it represent interest expense.
When you entered a negative number for code Z, it reduced your QBI, so that is why your tax went up.
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