There seem to have been a lot of issues with external monitors with Big Sur. The thread below seems to show issues. Are they fixed in the current 11.4 ?
I have a 2018 intel i7 6 core MacBook Pro
forums.macrumors.com
I have a 2018 intel i7 6 core MacBook Pro
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Some Users Having External Display Connection Issues With macOS Big Sur 11.1 and 11.2
In recent days I have seen something new. From time to time, my external display turns completely black as if the onboard graphics adapter performed a hard reset on the external display. And unfortunately, I can only return to 4K resolution when performing a fresh reboot.