The 5v reference with the sensor unplugged will never change on a properly functioning vehicle at any temp. What does change is the resistance of the sensor with engine coolant temperature. As this sensor resistance changes that causes the voltage reading at the sensor connector to change as the engine temp changes because it is part of a resistor divider back at the ECM Where the 5v ref is generated and the other fixed resistor resides. The fact that you get 5v on an unplugged sensor harness says that the two reference wires are not broken but they can be compromised. The current of attaching a load can manifest a wiring fault (on either wire) that you don't yet see with an open connector and no load. The sensor can also be faulty or wrong hence the resistance measurements and lastly the ECM can be faulty (very unlikely). Also, the ECM powers and grounds can have issues.

George