Big Sur "sleep" does not lock screen (when slept via keyboard shortcut)
Solution 1:
Maybe the settings got corrupted when upgrading to Big Sur.
- Open System Preferences -> Security & Privacy
- In the General tab, disable/untick "Require password after sleep"
- Quit System Preferences to have the changed settings written to disk
- Open the same panel again and enable the setting
- Quit System Preferences (and hope for the best)