I'm the OP and very much appreciate the feedback and ideas.
Please keep in mind that the room warms up fine from the heat pump. The problem is that it cools too much/too quickly between heating cycles. Could be an hour or several hours between cycles. The rest of the house is fine but when you walk into that one room, it's cold.
Data loggers were placed in the house to monitor for a couple of days and is easy to see what's going on in that room compared to the main living area.
Early on, the main feeder trunk in attic was enlarged to that area and registers adjusted to maximize air flow into that room. Insulation was checked and increased. Fans operated, door open/closed. Air flow re-balanced. Nothing really changed. No trouble with the A/C during the summer, just cold during cool or cold days.
I have been asked by the builder and I think that the cause is either cold air infiltration into the ducts in the cold attic which then slowly drops into the room below via the vent, (remember warm air rises and cold air drops), or air leakage from those big windows or a combination of the two. No air leakage testing has been performed.
Over the past two years many of the ideas suggested above have been tried (have to look into the paint idea) This has been a continuing warranty item since day one. Warranty department will do no more to find out why. Let alone who pays for the fix.
It looks like I'll have to have a permanent supplemental heater installed as some have suggested. Not the end of the world, but I really shouldn't have to.
To those who have seen the heating and cooling engineering that goes into each individual house, you have to wonder how this could happen. If you haven't, you can look up your address here. The "MAND" and "MANJ" are the calculations that Tuccillo was talking about earlier.
https://etrakit.sumtercountyfl.gov/e...ch/permit.aspx
Permit Search> Site Address> Contains> (your address)
Click the "Attachments" at the bottom of the first tab.