The problem occurs in both modes. In either mode the entry is processed the same way and the result repopulates the field. Unfortunately, the result of the processing is an inappropriate blank entry.
You got a canned response from the TurboTax rep that does not apply. This problem has nothing to do with which operating system you have. It's a problem within TurboTax itself.
What TurboTax has said regarding Windows 7 but has not made clear is that if there is a problem with 2019 TurboTax that traces to the fact that you are using it with Windows 7, there might not be any remedy. I'm not aware of anyone who has actually encountered a problem with TurboTax that is caused by using it with Windows 7.
As it stands, you probably have not yet received any Form 1099-B with box A or D marked and an incorrect basis indicated, so simply enter the correct basis amount in box 1e for now. Mark the entry as Estimated. In cases where the Form 1099-B does not have Box A or D marked you should be entering the actual cost basis in box 1e anyway, not in the box for corrected basis (per the instructions for Form 8949).
