TurboTax is behaving correctly. The catch-up contribution limit certainly is required to be pro-rated for the number of months that you were HSA-eligible in the same way that the regular limit is pro-rated. If you were HSA eligible for only 9 months of the year (and you were not HSA-eligible for December), your catch-up limit is $750. Section 223 of the tax code requires that the annual catch-up amount be added to the regular annual limit and then the sum divided by 12 to determine the monthly amount for pro-rating.
