iPhone 14 beeps at midnight after 18.5 update

Are you using a Sleep Focus? If so, then nothing should come through, if the Focus is on at midnight. If something does, then you can check to see if there is an app listed in the Focus settings that is allowed to provide notification. If you check the Notification Center, which when the phone is unlocked, is accessed by swiping downwards from the top of the phone. These are all past notifications. Does something appear here after you have heard. this notifcation?

Hello!

Force close all your apps, by swiping up from the bottom of the screen, pausing with your finger still on the screen, then swiping the apps up as they appear.

Then force restart your iPhone: Force restart iPhone - Apple Support

Let us know if the issue persists.

Do you by chance have a focus set up, which can be turned off in control center. See this --> Set up a Focus on iPhone - Apple Support