Since you are referring to your EV credit as 25% personal and 75% business, you apparently have self employment income that you must pay self employment taxes on. Your credits cannot offset the self employment tax so you must pay it regardless of how much credit you are eligible for. You are apparently offsetting all of the tax liability from your taxable income (excluding your self employment tax) and that is why the child tax credit goes down when the EV credit is entered. You are probably getting a refund of all your estimated tax payments except for the amount of estimated tax payments needed to pay the self employment taxes. You may also be getting some additional child tax credit on line 17b to supplement the child tax credit you lost when entering the EV credit.
