No, TurboTax did apply the last-month rule, but for tax year 2021. The amount is in line 18 because on your 2022 return, TurboTax asked you a series of questions when it realized that you did not have HDHP coverage for every month in 2022 (as you were doing your return). TurboTax can't calculate the income for the last-month rule for your 2022 tax return until you do your 2023 tax return, because TurboTax won't know until the end of 2023 if you "failed" to maintain HDHP coverage.
In any case, your description doesn't make sense unless you did not have HDHP coverage for every month in 2021, and since as we know, you did not have HDHP coverage for every month in 2022, then TurboTax would have done the last-rule penalty processing in tax year 2022.
Please tell me if this is not consistent with the facts, but I am thinking that line 18 is for the last-month rule processing for tax year 2021, not this year (2022).
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