Exactly what I did as a work around to this "bug" e.g. " setting the cost basis to $0 on the IRA Information Worksheet."
Also this is my experience:
"Oddly, I didn't have this problem with TurboTax from 2012 through 2019, even though it was keeping track of the cost basis of my other IRAs, but this did not affect the inherited IRA calculations. Apparently this bug was introduced in the 2020 version."
Hoping they fix this bug it can be very misleading as it causes taxable income from inherited IRA to be really under reported.
