@bos3
You would only lose eligibility for 2020 if you are following the “last month“ rule. The last month rule says that you are considered eligible for the entire year as long as you have coverage on December 1 of that year and you maintain coverage for all of the next year.
If you are eligible for all of 2020 because you had eligible coverage for the entire year, then you are not relying on the last month rule.
Let’s suppose you had eligible coverage starting September 1, 2020, but you contributed a full $3550 or full $7100 because you were relying on the last month rule, and then you lose your coverage in 2021. You are still eligible for the 4 months when you had coverage, but you lose the last month rule, so that in this example, 8/12 of your contributions become ineligible.
Eligibility is determined by your insurance coverage on the first day of each month. If you are covered by your parent‘s eligible plan on May 1, and then you enrolled in a different eligible plan as of June 1, then you have maintained your eligibility, even if there was a break between your birthday and June 1 while you arranged new coverage. If you did not obtain new coverage until June 15, then your HDHP eligibility begins July 1, and since you had a one month break in 2021, you would lose your qualification for the last month rule )if you were relying on it.)
