macOS Big Sur adds Battery to System Preferences for laptops. Settings previously found in Energy Saver are now found here. Turn OFF the setting for Prevent Computer from Sleeping for reliable Instant-On.
Prevent Computer from Sleeping must be unchecked for Instant-On to work reliably. However the checkbox does not work reliably. If it sticks (still checked when you re-open System Prefs) try clicking Restore Defaults.