There are no known issues with the Form 1116, line 18 calculation.
As you suggest, when you have qualified dividends (or capital gains), adjustments may be needed before accounting for those amounts on line 18.
Please see the Worksheet for Line 18 (Worldwide Qualified Dividends and Capital Gains) and the rest of the text pertaining to individual taxpayers under Line 18 in the Instructions for Form 1116.
If your calculation doesn't match what TurboTax is providing, please provide more details, referencing the worksheet or other relevant portions of the instructions, if possible.
