AB01 stalls in Bootloader

I am programming a AB01 unit with Arduino software.
When I upload a program, it gets uploaded

Initialising bootloader.
Silicon ID 0x256a11b5, revision 0.
Verifying rows.
Array 0: first row 34, last row 511.
Starting upload.
Uploading ( 10 / 100 )
Uploading ( 20 / 100 )

and then it runs smoothly:

Copyright @2019-2020 Heltec Automation.All rights reserved.
bootloader Rev 1.0
= = = = =ENTERING SETUP = = = = = = 

Then I unplug and plug back the AB01 and it gets stalled after bootloader:

Copyright @2019-2020 Heltec Automation.All rights reserved.
bootloader Rev 1.0

(and nothing more…)

What may be happening? Did I burn the Bootloader by mistake?
Also, I should underline that another AB01 runs correctly with the same program / configuration / …
It seems the problem is strictly limited to the “faulty” unit.

Any clue?

Can the program be downloaded normally after entering the bootloader mode?

I can download the program normally, again and again, but I think I am not in “bootloader mode” then.
I must plug the USB port, download the program and let it run.
If I unplug the USB port, or if I reboot the unit, I see that it stop after writing the messages I mentioned.

To relaunch the unit, I must download the program again (so the unit must keep connected to my PC all the time…)