I wonder if the generation of the CP503 was already in process before the explanation you provided was considered. The IRS states that it can take 30 to 60 days to process the explanation. In any event, since your nonqualified deferred compensation was properly reported on your 2022 Schedule 1 line 8t, it seems that you'll need to continue to pursue this with the IRS.
Even if this income had been reported on Form 1040 line 1 instead of Schedule 1 line 8t, it shouldn't change your taxable income. Apparently the IRS examiner is counting the deferred compensation twice by thinking it should be on Form 1040 line 1 without removing it from Schedule 1 line 8t.
Given that you received the CP503 only two weeks after mailing your explanation, I wonder how long it took you to mail that after receiving the CP11.
