Failed to create nested child process when python exe built using auto-py-to-exe on Ubuntu

Question

Below "main.py" Python program works ok when I run it using python3 (v3.8.10) command on Ubuntu v20.04.4 LTS.

import multiprocessing as mp

def P3_sm():
    print("P3 created.")
    print("P3 destroyed.")

def P2_sm():
    print("P2 created.")
    P3 = mp.Process(target=P3_sm)
    P3.start()
    P3.join()
    print("P2 destroyed.")

if __name__ == '__main__':
    mp.freeze_support() # ADDING THIS LINE DID NOT HELP!
    print("P1 created.")
    P2 = mp.Process(target=P2_sm)
    P2.start()
    P2.join()
    print("P1 destroyed.")

I built an exe using auto-py-to-exe v2.33.0 which formed below command and generated exe. pyinstaller --nonconfirm --onefile --windowed "/home/max/test/main.py"

When I run this exe it only prints below 2 lines and never terminates the program. Any help is highly appreciated!

P1 created.
P2 created.

When I run the exe file on Ubuntu platform I'm expecting it to print below 6 lines and terminate the program as it does when I run the main.py file using python3 command.

P1 created.
P2 created.
P3 created.
P3 destroyed.
P2 destroyed.
P1 destroyed.

Please note I added below line in my program and that did not help to resolve the problem on Ubuntu platform.

mp.freeze_support()