1 register blowing cold air, all others blow warm

[squishfatty]

Hello!
I had an inspection completed on a 1957 on-grade slab ~1200sq/ft house.
The house is square. The furnace is located in a utility room adjacent to the main square living space on the west side.

The seller's declaration mentioned that a bedroom was cooler than the rest of the house.
The inspector found that the register was blowing cool air when the furnace was on, while all other registers were blowing warm air.
He suggested it may be a disconnected duct or that the register is at the end of a run.

The room in question is located on the east side of the house in the middle (north south).
A bedroom at north east and the master at south east both receive warm air.

We have only a few days to make the decision.
Would anyone have some suggestions as to what the problem may be and the cost associated with fixing this problem?
Would it be best to have an HVAC specialist inspect the duct with a endoscope?

Thank you in advanced for your help!

[Admin]

The fact it is blowing air suggest the duct is still connected.  Could it be the duct in question passes through the garage ceiling before reaching the room?  I suspect at some point the duct is exposed to outdoor temperatures and may not be adequately insulated.  Put a phone playing music in the boot upstairs and find where it connects to in the basement.  You may be able to relocate it but it does sound like it's losing heat somewhere.

[squishfatty]

Thanks for the response!
The ducts are all under or in the slab. No attic. Sorry I should've specified that this is a bungalow with a vaulted ceiling.
What steps can be taken if this is an insulation issue?
Thanks!