I appear to have uncovered a bug and / or odd CT hardware behavior.
I had a 3.5mm mono extension cord plugged into my CT head unit. The HR sensor dongle was _not_ connected, just an unterminated extension.
Apparently the cord makes CT think there’s a HR receiver connected. Under that condition, Zwift comes up with CT enabled on the COM port for power, cadence, load & HR. If I unpair just the HR, I lose communication with the power and load. I can not make Zwift do CT sensors a la carte if it detects the sensor.
However, unplug the extension cord from the CT head unit and Zwift happily loads the ANT+ HR sensor!