The last-month is automatically invoked by TurboTax if you have HDHP coverage on December 1 of the previous year (12/1/2019 in this case).
It appears that you did not have HDHP coverage for every month in 2019 - is that right?
Yes, once you use the last-month rule, then you must pass the "testing period", which is basically the next year under HDHP coverage. If you don't then TurboTax asks you about your 2019 coverage and your 2019 contributions and recalculates what you would have been allowed as a annual HSA contribution limit if you had not used the last month rule. This is added to income on line 8 of Schedule 1 (1040) and an additional 10% penalty is calculated on the 8889 and added somewhere on Schedule 2 (1040).
You said, "We were on the HDHP plan for 10 months in 2019 and I contributed the appropriate pro-rated amount." If so, then you would not have owed anything for the last-month rule, so I would like to know what kind of coverage you had in 2019, how much you contributed in 2019, and was it really 10 months of HDHP coverage in 2019.
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