I have the same issue that the clamshell-mode was not working any longer when i updated my mac from 10.14 (Mojave) to 11.5.2 (Big Sur). I'm using a Kensington Dock SD4780P with 2x 4K-Samsung-Screens connected via HDMI.
Actual Work-A-Round is to use Amphetamin as mentioned in the thread before.
A much bigger problem for me is, that my network-connection is interrupted for just seconds multiple-times per day, what makes remote-working with VPNs or in Virtual-Machines a nightmare.
