I’ve been having the is issue now for several year where dialing the temperature change inside the car yields either HOT or COLD only. l
It would appear that the thermostat does not work at all.
THe problem is most noticeable during winter driving. Anything below 69 or 70 degrees on the dial is Air conditioning cold. As in maximum cold, set it to 60 cold.
Anything about 70 is HOT, as in maximum hot.
There is no variation of temperature
Any thoughts as to what this may be?
Faulty thermostat, stuck blend door?

That's a weird one. Does it misbehave on both sides, or only the driver's side? Usually sensor problems won't cause a transition at a middle temperature, it will be more like blowing warm until you set it for 60~61. But if the mix doors weren't working you'd get little or no change at one end of the temperature scale. If it's only the driver's side that misbehaves, it could be a mix door servo with a bad position pot. If it's both sides then that's a new one. Maybe a bad control head, because sensors don't behave that way.

I’ll have to check as I always notice it while I’m driving alone. I think that its not consistent meaning sometimes the thermostat behaves as expected, so that a PITA for when I bring it in.

Put it in dual mode and see if the passenger side changes temperature normally. If so, there's a good chance you simply have a bad driver's mix actuator. That isn't great news, the actuators on a QX50 are pretty much all 2-hour PITA's, but it's better than some other things.