When using this code:
import krpc
conn = krpc.connect()
vessel = conn.space_center.active_vessel

vessel.auto_pilot.engage()
vessel.auto_pilot.time_to_peak = (10,10,10)
vessel.auto_pilot.target_heading = 90
vessel.auto_pilot.target_roll = 0
vessel.auto_pilot.target_pitch = 10

vessel.auto_pilot.wait()

If used with any of the larger planes, they yaw fully to the left and go round in circles. As you suggested in an update, I set time to peak for 10 seconds using the Mallard, yet it still yaws all the way to the left.

This is what happens as the aircraft passes about 10 m/s:
https://steamcommunity.com/id/chickenEmpire/screenshot/794235968237514468

In fact it is a algorithm bug mentioned in #571, I have always met this problem when used a rocket like a real one in realism overhaul