CE BJT amplifier phase shift
What is the reverse near 10 kHz?


Try rescaling your Y axis to centre it around -180 degrees. There is nothing anomalous if you consider that the transition changes from -180 degrees to +180 degrees. It's just multisim thinking that the lower display boundary is getting a bit too close and rescaling -180 degrees to +180 degrees so it can adequately display the phase change at frequencies higher than 10 kHz.