Update: TurboTax confirmed today that that this is a flaw in their 2018 online version. It does not zero-out provincial tax credits correctly if those credits are unchecked, and it is not possible to access the forms using the online version to manually correct this programming error. My only remedies are to start from scratch using the desktop version (extremely time consuming for us as we itemize self-employment and medical expenses), or to print as-is (with the error) and file by mail, and have the CRA correct our filings themselves. Neither options are good in my opinion. Posting in case anyone finds themselves with the same problem.