OnePlus
Change Lock Timer After Sleep Mode auf OnePlus
OnePlus phones use a short grace period between the screen turning off and the lock actually engaging. This is controlled by a separate setting from the screen timeout, and it is easy to overlook because the display already looks off.
The setting responsible for this is Automatically lock after sleep. By default it is often set to a few seconds instead of immediately, which means the phone can be woken up and unlocked without any security check during that window.
To change it, open Settings and tap on Security & lock screen. Inside that menu, open the second Settings entry and tap on Automatically lock after sleep. Then choose an option from the list, such as Immediately, to remove the delay entirely.
Setting this to Immediately closes the gap completely: the phone locks the instant the screen goes dark. If you use a smartwatch or trusted-device unlock feature, keep in mind that a very short lock delay can occasionally interrupt those handoffs, so test your setup after changing it.
This setting only affects the delay after sleep. It does not change your screen timeout, and it does not replace the need for a strong unlock method like a PIN or password.