Nest thermostat has a mind of its own

[Camper-11]

Hello all.

I have had my Nest thermostat for many years.  Never had this issue and it's only happening on my upstairs unit.

Heating is gas.  A/C is an outside unit.  So, it's not a heat pump.

It's happened several times this spring, but never before.  I'll use today as an example.

Mode was set to "off" for a few days as the outside temp was reasonable.  However, it's supposed to get cool / cold tonight and tomorrow so I set the mode to heat.  Not Heat-Cool.

But right now, it's 77 inside and the heat is set to 70.  No big deal.  If it falls below 70, the heat will automatically come on so we don't get cold overnight..

Couple hours after I make the mode change to heat, I hear the HVAC running.  I check the app and it shows the air conditioning is on.  I check the mode and nothing is selecting now.  It's like it has automatically decided to make it whatever temp it wants.  

I change the mode to heat again and it turns the HVAC back off and mode is now again set to heat when I look at where the little check mark is.

I would say it sounds like it is set to heat-cool but it definitely isn't.

???  Any ideas?  Thanks in advance!

[Ddaniels]

I have this issue too but mine is changing from a/c to heat when I want it to stay on a/c. During the night while sleeping, a/c was on and set to 70. I wake up at 5am sweating and the heat is on and its 76. My Eco is off and I don't have anything programmed in my schedule. I can't figure out why it's doing this or how to make it stop. So frustrating!!