Why does my Jeep cool down while driving if thermostat is bad?

My questions are at the bottom.

So I have a 2011 Jeep Grand Cherokee 3.6 my girlfriend told me the car overheated at the Dunkin drive through but when she drove her coolant temps came back down

So I checked her coolant, checked its quality let it run made sure the belt on the water pump was good and nothing slipping, checked the fan to make sure it kicks on and also looked at all the hoses and radiator.

I let it idle in the driveway to see if I could replicate it and noticed the coolant temp rises pretty quickly up to about 280°f

I’m 90% sure it’s a thermostat I can feel the overflow and upper radiator hoses are still cool.

My question is if she has a bad thermostat (they’re $25, so I’m gonna change it anyways. How is her engine cooling-off while driving? 

I work on trash trucks and pretty rarely do you have overheating issues with them, so I’m not entirely sure if the thermostat is supposed to block the coolant entirely, or maybe it’s opening too late. I would think it would just stay hot or cold, Google isn’t helping, curious if anyone knows here.