I'm also having the same problem. I updated to Big Sur 11.3.1 and no longer able to clamshell mode. I have dual screen setup and able to clamshell before the update. I noticed closing the lid under Diplay settings the actual macbook screen still shows as active screen and also all my monitors flicker every few seconds. Hopefully there's alternate solutions to go back to clamshell mode on Macbook. Thanks
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