My only comments were concerning the desktop CD/Download editions, looking directly at the Form 8959 you referenced to Line 20. Line 20 pulls from Line 1 of the Form 8959 which pulls directly from box 5 of the W-2.
The online editions are another issue and you can contact TurboTax support directly to confer with them concerning the online editions for the Additional Medicare Tax.
Use this website to contact TurboTax support during business hours - https://support.turbotax.intuit.com/contact/
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