While at idle with temperature in the normal range, the fan is not running or just running a low speed, so either no cold air or very little cold air is being sucked thru the radiator, which means no or little drop in temperature.
When you go down a hill, the cold air is being forced thru the radiator fins due to the speed and will rapidly decrease the coolant and engine temperature.
This is actually normal and does not indicate a problem.
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