I'm seeing the same, It's grey out the option and doesn't allow the user to change the setting and it doesn't actually set Fast User Switching to be enabled. I was able to add a device as an exception to the configuration policy, and then set Fast User Switching and then remove the exception. Not a solution but a workaround if you need this on.
It's currently a product issue is what we got told by support. I guess more pressure needs applying to actually get a fix.
"PI-009059 - Computers with macOS 11 or later fail to respect the Enable Fast User Switching setting in the Login Window payload of a computer configuration profile."
Here some commands that DONT work:
defaults write /Library/Preferences/.GlobalPreferences.plist MultipleSessionEnabled -bool true
defaults write ~/Library/Preferences/ByHost/com.apple.controlcenter.plist UserSwitcher -int 18
defaults write ~/Library/Preferences/com.apple.systemuiserver.plist menuExtras -array-add '<string>/System/Library/CoreServices/Menu Extras/User.menu</string>'
When I run the commands manually - it works. Pushing a script with Jamf failed, haven't been able to test why. Using Monterey with M1 iMacs