2018 Prius Temperature Warning Light turning on

Recently I noticed that when I am driving to work in the morning and I have the heater/defrost on my temperature warning light will turn on when I come to a stop and disappear as soon as I start driving. I took it to the dealership for 100k maintenance and asked them to check if I had a coolant issue related to this. They said they were not able to find any leaks and my coolant is full. I had them test drive with the heater on and they said they did not see the light come on. I was reassured everything seemed fine but then two days later the light reappeared when I was on my way to work. I am worried I am messing up my engine and am not sure what else could be wrong.

 

I think they did but they said they couldn’t recreate the issue and cause the light to come on

 

I was driving this morning no heater or defrost on and it sadly came on again so I don’t think the issue is related there. The dash temperature read 55 degrees Fahrenheit and it does seem like this issue is restricted to the morning when it’s cold

 

I checked and it’s full, they made sure it was full when I was at the dealership last week as well