Fresh install LE 12 on Raspberry PI4 deliver no HDMI0 output when connected to UHD TV (3840 x 2160).
Comparing LE 12 /flash/config.txt with a working LE 11 show one line difference:
hdmi_enable_4kp60=1
When commenting it (#), LE12 put video on HDMI0 and my TV show it.
So the problem is above line in a config.txt that must be disabled on Raspberry Pi 4 devices. I do not know about RPI 5.
Developers please consider that change in a next update for the LE 12 image for a RPI 4.