Big Sur "sleep" does not lock screen (when slept via keyboard shortcut)
Maybe the settings got corrupted when upgrading to Big Sur.

Open System Preferences -&gt; Security &amp; Privacy
In the General tab, disable/untick &quot;Require password after sleep&quot;
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)