Remote Management on Big Sur has a strange bug(?). When you enable & configure RM via UI it will work as intended, but as soon as you try the same via command line, things will go south. Why would you do that? You are most likely virtualizing MacOS too.
You have probably run something like this to enable Remote Management:
cd /System/Library/CoreServices/RemoteManagement/ARDAgent.app/Contents/Resources/
sudo ./kickstart -configure -allowAccessFor -specifiedUsers
sudo ./kickstart -configure -allowAccessFor -allUsers -privs -all
sudo ./kickstart -activate
Note that the above command(s) grant all privileges to any user who was able to connect
Now try to connect from an other machine via the Screen Sharing application: you will probably end up with a black screen.
To fix the issue, you will need to reset Screen capture permissions, so take a note of the currently whitelisted applications (Security & Privacy > Privacy tab > Screen Recording).
Run the following command, then reconnect:
tccutil reset ScreenCapture
You should be able to see you Big Sur desktop as intended.