Just experienced this issue when filing my 2023 tax return today. By default, it checked all months of the year. But when I navigated to the next page then back, everything except January was unchecked. Obviously there is some bug.
My suggestion would be to try navigating to the pages following the months selection then back. Or to uncheck and recheck the checkboxes of all the months you had insurance to work around this bug.
-- speaking from my experience as a software engineer
