No, this is not a bug with TurboTax. Based on this, line 4d represents your total itemized deductions for 2025. Because of that, line 4c for the period of 1/1/2025 to 8/31/2025 cannot be greater than your itemized deductions for all of 2025. If all of your itemized deductions were taken by 8/31/2025, the entry on 4c will have to be equal to 4d.
When you use the annualization method to reduce or eliminate a penalty, keep in mind that each of the columns represents a time period from the beginning of the year to either 3/31, 5/31, 8/31 or 12/31. For instance, in your screenshot, line 1b is zero - unless you had a loss of all the income from line 1a for 1/1 to 3/31, then 1b for 1/1 to 5/31 should usually be equal to or greater than line 1a (barring a loss of income).
You can't employ the 7.5% AGI limitation for medical expenses to justify having an increased medical expense deduction for a given time period for purposes of this form. You will need to divide up your medical expense deductions for each of the time periods based on the total medical expense deduction you got from Schedule A line 4.
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