In 2019, you did not use the last-month rule*, because you had coverage every month.
In 2020, you DID potentially use the last-moth rule* because you acted as if you were able to use the full annual HSA contribution limit even though you did not have HDHP coverage for each month free of any conflicting other coverage.
In 2021, when you do the HSA interview, TurboTax will realize that you did not have HSA coverage all year. So it will innocently ask you what type of HDHP coverage you had on December 1, 2020.
Based on your answers, TurboTax will suspect that you used the last-month rule* in 20200 (and you did), so will ask you a series of questions that will result in TurboTax figuring out your allowed contributions without the last-month rule* and adding it to your Other Income along with a penalty.
You got the message "good news, you don't need to include any income under the last-month rule" in 2020, because you in fact didn't use the last-month rule* in 2019 - you had coverage every month.
You will fail to maintain HDHP coverage in 2021, however, and thus will answer the same questions but this time not get the good news.
*Last-month rule
The last-month rule, as it seems that you know, is a special rule that allows to use to the full annual HSA contribution limit even if you did not have HDHP coverage for each month, so long as you had HDHP coverage on December 1 of the tax year. In short, this is a retroactive adjustment to your HSA contribution limit to make it possible to act as if you had HDHP coverage each month, even when you didn't.
The only catch is that you must keep HDHP coverage during the "testing period", usually the next 12 months. If you fail to maintain HDHP coverage during the testing period, then your actual HSA contribution limit is calculated based on your information without the last-month rule, and any excess amount of contributions under the revised contribution limit will be added to Other Income and a penalty assessed.
It is correct that you did not have this failure to maintain in 2020 because you didn't use the last-month rule in 2019, but since you did in 2020, it will catch up with you on your 2021 return.
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