this is driving me nuts...
i used the 7" display with my pi 3b's for a long time without issues.
but when i replaced my 3B's with a bunch of 4B's the display started showing a weird behaviour which i couldnt resolve until now:
the pi COLD boots perfectly fine every time... but no matter what i try, if i reboot the display stays black while the raspi boots.
and yes, it boots.. i can not only see the file access led flashing - if i connect a hdmi cable after such a "black reboot" the console contents are shown on the tv set.
with priority to the lcd enabled in config.txt it also cold boots fine into the 7" display even if there is a hdmi cable plugged in. but as soon as i reboot, the system starts booting into hdmi instead.
by default i boot into console btw. i have tried enabling and/or changing some of the display related boot options in config.txt but neither changed anything.
i also installed the new sc card imager tool and installed a fresh raspbian full desktop to a new sd card and it behaves the same.
so since upgrade to the pi 4b's i am no longer able to reboot my pi/7" display combo. i have to shutdown, switch off and switch back on whenever i do any changes or updates the normally would require just a simple reboot.