Mac mini 2018 displays go black in certain reboot phases or in QuickTime Player Full Screen mode

Error between Mac mini 2018 and displays like LG UltraFine 4K 23.7" Thunderbolt 3 (24MD4KL-B) display connected via Thunderbolt 3 USB-C cable or LG 55" 4K UHD OLED Smart TV OLED55B6V connected via HDMI:

When the Mac mini 2018 boots into Command-R Recovery mode or the macOS installer reboots in Option-Command-R Internet Recovery mode, the display suddenly go black. Disconnecting and reconnecting the USB-C Thunderbolt 3 or HDMI cable reveals what is on the display and what user input is needed to continue. Also Shift SafeMode boots into a black screen.

Via Thunderbolt 3 to the LG UltraFine 4K 23.7" (24MD4KL-B) display that error can be reproduced almost always. Via HDMI to the LG 55" 4K UHD OLED Smart TV OLED55B6V that error does not happen so often (more often if the Mac mini is shutdown, HDMI cable disconnected & reconnected and then Command-R startup, for example. Occasionally even a normal reboot boots into a black screen via HDMI).

Likewise via both Thunderbolt and HDMI, when QuickTime Player in full screen mode after a few seconds fades its video playback controls, the whole display goes black until the mouse is moved. In SafeMode boot this error does not happen.

Resetting PRAM via Option-Command-P-R, resetting SMC or installing LG Screen Manager 2.32 (LG UltraFine 4K has the latest 3.00, 5249,4.13,0.3 software) did not fix the error.