If you are finishing construction, then it's still a construction loan, not a home improvement loan. Construction loans are a "Purchase" for HMDA. Since you don't know for sure if this will be replaced by another loan and since it will be paid from the sale of the home, I would call it a short term purchase and report it. Things could change, but you have to make a call one way or the other now.
You could say it will be replaced by the 2nd 18 month financing. If that route was chosen, the first loan would be a temporary loan and the 2nd loan would be reported as a purchase.
Pick one, document why and report one or the other.