Why does my Toyota 2013 Sienna accelerate on its own when the engine is cold?

The outside temperature was below 32 and it was close to 25 or so. When I was driving the car at 15 mph with engine temp still slightly above cold(not at optimal which is almost half way) and slowed down by using gentle brakes(not hard brakes). Just before the vehicle comes to a full stop, there's unintended acceleration and I could see even the rpm(scale) to spike temporarily. This has been happening every time when outside temp is cold and vehicle is driven with cold or slightly above cold engine temperature. Please note: this problem disappears when engine temperate is at optimal and I have been driving a Honda Civic for the last 7 years(was bought new ), I never had experienced this problem in that car even when driven with cold engine. What could it be? Car is still under warranty so any advice would help.

I'm sure that it has a fast idle when the engine is cold, thus the "unintended acceleration" all fuel injected cars have a fast idle when cold. Why don't you consult with the dealer what the cold idle speed should be?

New problem or old problem? In any case the car is having trouble deciding whether it should be on fast idle for cold engine or slow idle for warm engine. You need to take it in for repair. If it is a new problem the engine temp sensor is probably going out.