I'm sorry for the frustration. I just tried this out and the password did transfer from 2019 to 2020 when I transferred a file. That probably means that whatever issue you ran into is more subtle - perhaps specific to some machine situation or something different with the return I used relative to yours.

Suggestion

- Try to transfer just one file again. Maybe this was an issue we had earlier in the season. I do know that password is the last thing we transfer in the return and that if password transfer fails we still transfer the file (not saying that's a good thing.. just saying that's what I think we do).  If the one transfers with password, then you might want to consider redoing the remaining ones assuming you haven't already modified them for Tax Year 2020

- If the password failed to transfer again, can you got to Help -> Send Logs and mail the resulting zip to proseries_engineering@intuit.com? I'll take a look.