I saw that 8960 warning and wondered what it might be.  Some hidden item in the 2025 tax legislation that applies to dividends from Saudi oil companies?  Asked AI and the most likely candidate there was, 

"Interpretation of "Properly Allocable" Deductions: Determining which expenses are properly allocable to investment income for NIIT purposes (Part II) can be complex and differ from regular income tax rules, leading to potential misreporting."

I decided it was some exotic situation and I don't have any exotic clients.  Just people who make a lot of money and keep much of it in the bank earning interest.