Echo Dot and Smartthings

No, it’s working exactly as intended. It’s simply user error. Auto mode is exactly that. You set the range of temperatures that you’re okay with and it will make sure it “auto” turns on the AC or heat to stay in that range. I have mine set between 67 and 76 when I’m home. When I’m away, it changes to 62 to 81. There hasn’t been a time where either of my Ecobee3 thermostats deviated from the set auto. This is also an exponentially more energy efficient way to manage climate. $50 electricity bill in a 4000+ sqft house anyone?
If you want the Ecobee to stay around a certain setpoint, then change it to cool or heat mode. As you are doing it now, look at the Ecobee itself when you’re changing the setpoint using Alexa. All it will do is adjust the min and maximum by however many degrees you’ve adjusted your setpoint. Let’s say your auto is at 65 min and 75 max, your setpoint at that time would be 70. You then tell Alexa to turn up the temperature 2 degrees, your min is now 67 and max is 77 with setpoint at 72. If you plan on using Alexa to adjust the Ecobee, then you’re gonna have to choose heat or cool. Alexa hasn’t been programed correctly to understand the auto mode that Ecobee has. SmartThings also did not handle it correctly when support for the Ecobee first emerged. Personally, I just about never touch my thermostat since I have my schedule set up nicely and I have 10 sensors tied into the Ecobees with Follow Me on and they talk to each other, too.
One complaint that people have is that the Ecobee is just too much for them to handle. The customizability of it is insane. If you haven’t ever gone to the full settings, you should take a gander. Go to the thermostat itself then go to Main Menu then Settings then Installation Settings and just peruse that. There’s page after page of settings you can change, one of them being the delta value for auto mode.

When sleep comfort setting is activated, Smart Home/Away is disabled to prevent it from automatically switching to away mode because it doesn’t detect anyone and you end up being way too hot or way too cold.