Standby mode reverts to lock screen / will not start at all

Standby has been working well previously, but it has started to fail. Used to be that if I placed the iPhone on the charger it would go straight into Standby mode, but now when I do that it behaves as if it is locked and will not display Standby without being unlocked. So I take it off, unlock it and place it back on the charger; it is then in active unlocked mode and so is waiting for input until it eventually goes to sleep and shows a locked version of standby with a small version of the time display top right. Then next time I look at it in the night it is either dark or showing the keypad again and wants to be unlocked before Standby will work. I have tried restarting, switching Standby off and on again. Software is up to date 26.3. iPhone 16 Pro Max.