Just spoke to an agent and in my case it wasn’t related to either the last month rule or the question about December 1 of the previous year. In my case (married filing jointly) I had to mark (check box) that we both have “health related accounts” though its just one of us who pays for it. In a family plan though one person is paying into HSA, both family members are on the account, so when you’re asked to check all the boxes that apply under both names, for the question “Tell us about the health-related accounts you had in [year]” make sure you check HSA for both members if not it assumes that you didn’t have an account or HDHP and asks why you had a break.
