@sistar_hh and every one who reads this in the future )))

If you set in your program:

Boot option set to 'Run main.py directly'
set_boot_option(0)

it will be executed by default on boot.

But! If you press G0 (BtnA) button as soon as the device is powered on, then the default UIFlow2 app will be started.

I've found it in https://github.com/m5stack/uiflow-micropython/blob/master/m5stack/fs/user/boot.py

"""
boot_option:
    0 -> Run main.py directly
    1 -> Show startup menu and network setup
    2 -> Only network setup

Quick reference:
    when use uiflow2.m5stack.com website, click RUN button to run workspace
    code, boot_option won't change, if you click DOWNLOAD button to download
    workspace code to device, boot_option will change to 2, it means after
    download code done, device will auto reboot and won't show startup menu, 
    only do the network connect, but after network connect success, you can
    still download or run workspace code. If you don't want do anything after
    boot, you can delete this whole file. If you want show startup menu again,
    you can hold BtnA(most device) and click reset button or repower device
    until show the startup menu(for those devices with screens, and this is a
    temporary method and may change in the future), after that the boot_option
    will change to 1, so next time still will show the startup menu.

    BTW, the network connection time has a default timeout (60s), you can modify
    the following definition to change this default value.
"""