IPhoneXR, after upgrading iOS 13.3. 1 Can't the screen lock automatically?

This can be adjusted. The method is as follows: Settings > Display and Brightness >; Automatic lock (I don't have my mobile phone, but I remember it clearly on the display and brightness)

If you are too lazy to do it one by one, turn on the low battery mode. Directly default to automatic sleep for 30 seconds. Settings > battery > low battery mode (if it is difficult, you can open it directly in the control center, which is the symbol of the battery).