Thanks for the help barczy
It turned out to be a faulty coolant temp sending unit. All is functioning correctly now. What had me puzzled was the gauge reading accurately up to the half-way (normal operating range) mark on the gauge but not showing any hotter when the 50/50 mix coolant was obviously way too hot. (Sorry, I didn't have a test gauge handy to read the actual coolant temperature).
Some of you remember the old days when sending units (coolant temp, gas tank level, throttle position sensor, etc) were constructed using a wire coil and a wiper contact that traveled along that coil which produced variable resistance depending on the contact position on the coil, in order to control the gauge needle movement. Sometimes it would get a break somewhere in the wire coil which would render the gauge functional only up to the point at which it got to the break. (e.g A gas gauge the functions normally from 'full' down to a certain point and then fall straight to read empty although there was still some measurable gas still in the tank).
Today's solid state construction no longer uses that coil / wiper contact arrangement. It is done using 'solid state' circuitry. I'm not familiar with the exact components used nowadays (obviously some type of variable resistor), but whatever the case may be, it obviously is capable of producing the same effect as the old coil / wiper configuration.
I spent most of my employed life working on industrial machinery but I did spend a few years as an automobile mechanic, including a Chevrolet dealership when I lived in Florida. In my recollection, I don't remember finding a modern 'solid state' coolant temp sending unit that exhibited this type of symptom, but I guess I've seen it now haven't I. LOL
As far as the labor difficulty ("murder") replacing this sensor, I didn't have any problem at all, even with me having only one good right hand and a prosthetic left arm. I was able to get a 19mm 3/8 drive deep well socket on the sensor fairly easy then used an 18" extension and a 3/8 drive swivel. Before installing the new sensor into the deep well socket, I inserted some wadded-up paper to take up some space which enabled the sensor threads to stick out of the socket in order to start the thread into the head. Removing / installing the sensor itself only took about 10 minutes. The whole job including catching the antifreeze in a drain pan and returning it to the recovery tank after the sensor was installed, then running the motor up to operating temperature (thermostat open) and adding coolant to the proper level took a total of about 45 minutes, all 1 1/2 handed LOL
Anyway, just wanted to post these details in case somebody else falls victim to a sensor that exhibits the same weird symptom as mine did.
Photos linked of the tools used and the approach angle, and also one showing the wadded-up paper I used to expose the threads out of the deep well socket.