Charitable donations that are claimed as an itemized deduction do not directly reduce AGI. Your AGI is calculated first, then itemized deductions are subtracted from AGI to get taxable income. So the donation reduces your taxable income. It does not reduce your AGI. That's the way Form 1040 works.
You said you expected to see the donation on "1040 10a." On Form 1040 for 2020, line 10a is not charitable contributions. It's other adjustments from Schedule 1. Form 1040 line 10b is for charitable contributions up to a maximum of $300 if you are not itemizing deductions. If you are claiming itemized deductions, Form 1040 line 10b is not used.
