If the gauge is accurate, this behavior is common to an air lock. These usually clear themselves if they have a clear pathway to purge. Remove the radiator cap and run the engine up to thermostat opening temperature, then elevate the engine speed to around 2k rpm and observe coolant flow in the radiator. If their was an air lock, you'll start with a full radiator and once the engine burps, the coolant level will drop so be prepared to keep it topped off. It also makes a difference if you're running an expansion tank to recover the coolant and maintain and fully topped up radiator.