Maintenance Running cold downhill

[Stuarthewild]

On my 2018 sr5 during a road trip, I noticed while going down steep grades that my thermostat needle would noticeably drop toward the cooler side of the gage.
It was more noticeable when I would use the trucks gears to help hold the truck at the speed limit.
Is this normal, or something I should get checked out? I've not had a vehicle that's done this before

 

[Junkhead]

Your injectors shut off when you are coasting down the hill in gear, it’s called DFCO. It’s normal for your engine to cool down a bit.