I don't see any indication that income from a taxable conversion would be subtracted from MAGI.

https://www.irs.gov/publications/p590a#en_US_2020_publink1000230489

(1) Was the Roth conversion taxable?  And is the taxable amount the only portion that is added to AGI/MAGI?

(2) Double check that you entered the conversion correctly.

(3) Does the Lacerte IRA Deduction Worksheet show how MAGI is calculated, or does it just show "MAGI" and you manually calculated MAGI to verify the numbers?  If it shows the calculation, could you post a picture of it here (be SURE it does not show any private information about your client)?