I think I am hitting a similar issue and have got to the point of "to disconnect the cable from the PC before rebooting to start new version of firmware."
I flashed MB204 on a T-Beam using espflash command line on top of a unit previously flashed with MB156. I pulled the USB immediately after flashing.
> espflash write-bin --after hard-reset --baud 921600 --before default-reset --chip esp32 --confirm-port 0x10000 ~/Downloads/SoftRF.mb204.esp32.bin
I plugged in the USB and it started up in MB204.
I pressed reset and it started up OK.
I pressed PWR off and on again and it started up OK.
I put in the battery with USB disconnected and it "bricked": > Sequence is red dim, wait, Blue bright + red still dim, wait, Blue still bright + Red bright, wait, off.
Pressing PWR on / off and trying reset all resulted in the unit being "bricked".
Inserting the USB with the battery installed resulted in the Blue CHG LED on - PWR Sequence is Blue bright + red dim, wait, Blue still bright + Red bright, wait, Blue still bright.
Any help appreciated....