I called support again today, and they "solved" this for me.
Here's what happened:
- they tried to transfer me to a team to help, but I refused to pay extra for 'live'
- they accepted that my issue was a bug; they gave me a license code for the desktop version of TurboTax
- they advised I would have to fix in 'forms mode', but they were wrong, as per below...
- I downloaded the desktop version (following their instructions to first create an empty tax return and select my state) and downloaded my tax2020 file from online
- the wrong tax figures displayed, matching the online figures
- I clicked into the Mortgage Interest section of Deductions & Credits
- I clicked through and made no changes, just selecting some items which it didn't know my previous selection for
- it updated to the correct tax figures, matching exactly the calculation I had done to file estimated taxes
So, this is a tax calculation bug in TurboTax Premier Online, and TurboTax Premier desktop edition does not have the same bug.
