Welcome to the forum, LJ2015!
What you describe is a feature, not a bug. It's designed to hold gears longer than usual when cold to help get the drivetrain and catalytic converter up to temperature faster.
If you have an outlet available, using your block heater will help. Also i5'll get the cabin warmer sooner, reduce wear, and help fuel economy a little. See thread: Use your Micra's block heater: save fuel, reduce wear, be more comfortable.
View my fuel log 2015 Micra S manual: 5.0 L/100 km ... 56.5 mpg (Imp) ... 20.0 km/L ... 47.0 mpg (US) ...