Hi,
I have similar problem, trying to flash a B450M mortar titanium for a 3600X, with no success :
I update the BIOS, ( the led next to the flash button blinks for 5min with cpu debug led also on ),
then when i boot the CPU debug led light up for 5 second, and then turn off, no other led is on, the fans keeps turning but nothing is happening on screen.
Tried several BIOS including 7B89vA7 and the latest 7B89vAB1(Beta version)
What BIOS version did you end up using ?
EDIT : Problem solved, made yet another try at clear CMOS + bios flash with latest available : 7B89vAB1
Thanks !