I agree with cruise controls may differ between cars, in my chevy downhill speeds are maintained, though I am pretty much a flatlander. Rented a Chrysler 300 for the drive from San Fran to seattle, set cruise, and it performed admirably downshifting when needed to maintain set speed. I enjoyed not having to use the brakes and turn off speed control, so there is at least 1 car that performs as I would expect on up and down mountain roads. Take a test drive in a new one and see if it performs differently.
Glad to see you got some comparative results and really glad you posted back with the results!
Here’s what I think could be going on. I may be all wet but you never know.
On a vehicle equipped with an automatic transmission, the engine is coupled to the transmission through a torque converter. Forgive me if you already know this but the torque converter is analogous to two fans placed close together, facing each other. The engine side fan blows toward the transmission side fan using a liquid coupling rather than air. The engine fan turns the fluid and the fluid turns the fan connected to the transmission thus connecting the engine to the wheels through this coupling.
The efficiency of the torque converter is pretty good but not as good as a direct coupling. So to get the best gas mileage, the torque converter has a mode that provides a “locked connection” that is enabled when the vehicle is cruising above a certain speed. This is commonly referred to as torque converter lockup.
When the converter is unlocked, the engine spins faster than the transmission to get the car to move. Conversely, if the car goes faster than the engine (ala coasting down a hill) the converter slips and does not force the engine to turn faster. The car can coast faster and faster as it descends the hill.
If the converter is locked up, then the engine will be forced by the transmission (which is connected to the wheels) to turn faster. But the engine has a lot of drag and the car will not coast very efficiently. This is known as engine braking.
Some manufacturers play games with the way their car reacts to hills or coasting in general. I have had a number of Corvettes that display the following behavior; when exiting an expressway, the car initially slows very rapidly as the torque converter is locked up and the engine is providing resistance. Once the engine speed falls below a certain point, the lockup is disabled and the car continues to zoom along for great distance, slowing much more gradually. This is a way to maximize fuel efficiency.
What I think may have been happening to your car is that the lockup function was sticking “on” when it should have been released. This can happen in older or abused transmissions when the fluid is not changed regularly or anytime something associated with this section of the car just isn’t working right. As people have pointed out, some cars do this type of thing intentionally. However, since you tested another, similar car and get the same results, I suspect the car was misbehaving up until that point it changed.
Does your car have a tachometer that you can watch to see the engine speed vs the speedometer for vehicle speed?