Pybullet example two-link robot applyExternalForce unexpected behavior

test.py

import pybullet as p
import pybullet_data
import time
import numpy as np

SIM_TIMESTEP = 1

# Init sim
physicsClient = p.connect(p.GUI)#or p.DIRECT for non-graphical version
# Set up the visualizer
p.configureDebugVisualizer(p.COV_ENABLE_WIREFRAME, 0)
p.configureDebugVisualizer(p.COV_ENABLE_SHADOWS, 0)
p.configureDebugVisualizer(p.COV_ENABLE_GUI, 1)
p.setRealTimeSimulation(0)
p.setTimeStep(SIM_TIMESTEP)

# load robot
startPos = [0,0,0]
startOrientation = p.getQuaternionFromEuler([0,0,0])
bid = p.loadURDF("test2link.urdf", startPos, startOrientation, useFixedBase=True)

# springy control for both joints
# p.resetJointState(bid, 0, 0.5, 0)
p.resetJointState(bid, 1, -0.5, 0)
p.setJointMotorControlArray(bid, [0,1], p.POSITION_CONTROL, targetPositions=[0,0], positionGains=[0.01,1], velocityGains=[0.1,0.99])

idff = p.addUserDebugParameter("Test force", -0.01, 0.01, 0)

while True:
    try:
        # Apply force to last link
        pend = [0,1.1,0.1]
        Fend = [p.readUserDebugParameter(idff),0,0]
        p.applyExternalForce(bid, 1, Fend, pend, p.WORLD_FRAME)
        p.addUserDebugLine(pend, np.array(pend) + 100*np.array(Fend), lineColorRGB=[1,0,0],lifeTime=0.1,lineWidth=2)
        p.stepSimulation()
        time.sleep(SIM_TIMESTEP * 1e-3)
    except:
        raise

test2link.urdf

<?xml version="1.0" ?>
<robot name="test2link">
  <!-- Body -->
  <link name="body">
    <visual>
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <geometry>
        <box size="0.1 0.1 1"/>
      </geometry>
    </visual>
    <inertial>
      <mass value="1"/>
      <inertia ixx="0.1" ixy="0" ixz="0" iyy="0.1" iyz="0" izz="0.1"/>
    </inertial>
  </link>
  <link name="link1">
    <visual>
      <origin rpy="0 0 0" xyz="0 0.5 0"/>
      <geometry>
        <box size="0.1 1 0.1"/>
      </geometry>
    </visual>
    <inertial>
      <mass value="1"/>
      <inertia ixx="0.1" ixy="0" ixz="0" iyy="0.1" iyz="0" izz="0.1"/>
    </inertial>
  </link>
  <link name="link2">
    <visual>
      <origin rpy="0 0 0" xyz="0 0 -0.25"/>
      <geometry>
        <box size="0.1 0.1 0.5"/>
      </geometry>
    </visual>
    <inertial>
      <mass value="10"/>
      <inertia ixx="0.1" ixy="0" ixz="0" iyy="0.1" iyz="0" izz="0.1"/>
    </inertial>
  </link>
  
  <joint name="0" type="continuous">
    <parent link="body"/>
    <child link="link1"/>
    <axis xyz="0 0 1"/>
    <origin rpy="0 0 0" xyz="0.0 0.1 0.5"/>
  </joint>
  <joint name="1" type="continuous">
    <parent link="link1"/>
    <child link="link2"/>
    <axis xyz="0 1 0"/>
    <origin rpy="0 0 0" xyz="0 1 -0.1"/>
  </joint>
</robot>