All the equipment is official Raspberry Pi, the PSU & mHDMI cables. Plus a fresh installation of Buster 2 days ago, which worked in my RPi3B+. Rebooted with the cable in the other socket, & got the expected blank. Rebooted, mHDMI cable back in HDMI0, & now I get 640x480, 4 raspberries, the logo, & ...