Did you mistakenly fail to indicate to TurboTax that the $18,500 distribution was converted to Roth? Failing to indicate to TurboTax that the distribution was converted to Roth would produce the same increase in AGI as would correctly reporting that it had been converted, but it would be seen as a distribution that is to be included on Form 8880 line 4.
You can either edit the 1099-R in TurboTax and make sure that you've explicitly indicated that this amount was converted to Roth, or you might consider deleting this Form 1099-R and reentering it, similarly making sure in the follow-up questions to indicate the amount converted to Roth.
