Temperture Gauge acting odd

The vehicle listed above is a 2 Door Sport_4.0L_SOHC_V6
118K miles-well taken care of and I really like it.


Situation:
I’m in my 70’s and don’t want to experience a vehicle breakdown at my age, so I’ve been taking my Explorer in for preventative maintenance for one thing at a time (fuel pump, starter, alternator, seals, gaskets, spark plugs & wires). Last week I had the original water pump replaced (which included an engine coolant flush). It had previously shown no signs of any problems, but... It was on my preventative maintenance list.

A couple of days later while driving I happened to notice that the temperature gauge/dial on my instrument panel as performing oddly.

Always before (20+ years), the temperature stem was on “C” (cold) when I first got in the car that day and then slowly warmed up to just below halfway on the temperature dial once it was warm. When I parked it for the day, I assume that the temperature stem slowly dropped back to “C” as the vehicle cooled off, because it was back at “C” the next day.

Problem:
Now I notice that the temperature stem still slowly warms up to the halfway point on the dial; HOWEVER, when I turn the engine off for the day the stem stays at the halfway point and is still stuck at the halfway point when I first get in the car the next day (?). Then, the next day, when I first start the engine, the stem immediately drops to the bottom of the dial, way below “C”. As I drive it then slowly warms back up to the halfway spot on the temperature dial.

I have gone back out to the vehicle several hours after having turned the engine off. The temperature stem was still stuck at the halfway spot on the dial. When I then started the engine the stem dropped down at that time to just below the “C”- the engine was cooler, but not all the way cool yet (apparently).

Summary Review:
The water pump was replaced, including an engine coolant flush, by the Ford Dealer.
Subsequently, the temperature gauge responded differently than usual- remaining stuck at the usual “engine at its warmest” midpoint on the dial.
It remained “stuck” there until the engine was started the next morning and then immediately dropped to the bottom of the dial (instead of dropping to “C” as it always had before). The temperature stem dial “sticks” were it is when the engine is cut off. When the engine is restarted the stem then drops, but apparently relating to how cool the engine is.

Question:
-What do you think happened during the water pump, coolant flush to alter the temperature dial/thermostat?
.
● Do you think the problem is the thermostat and/or the coolant temperature sensor?
● A related electrical connection/short? (The dial sticks where it is when the engine is turned off and then drops to the bottom of the temperature dial once the engine is cool and then started again).

-Should I have the Dealer replace the Thermostat and Cool Temperature Sensor and see what happens (‘shotgun’ first step I’m leaning at), or should they diagnose the problem otherwise before doing that?

Thanks very much, (video attached)
Michael P.

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