how to get rid of dark rectangle on lockscreen behind the clock

When you open your home screen, I'd guess you have a large widget on that portion of the screen. Do you? What you're seeing is likely caused by having Reduce Transparency turned on in Settings > Accessibility > Reduce Transparency.

Hey, started having the same issue, dark rectangle randomly appeared on my lock screen without me doing anything. What the...?

Solution: Restarting my phone got rid of it but seems like some bug in the newest iOS.

I wouldn't expect an iOS update to change this for you. If this was an iOS issue, everyone would have the same issue and this forum would likely be flooded with people complaining, but they're not.

Try Force Restarting your iPhone. Follow the directions here exactly --> Force restart iPhone - Apple Support

Have you tried to change the font size and color ? I'm wondering if it's not an artifact of the current font type and color.

I'm sorry, but I'm out of suggestions. You might want to reach out to Apple Support or visit your Apple Store and let them take a look for you.

I still think it's tied to the widget you have on your home screen. Can you remove or relocate the widget to see if makes a difference?

I just took a moment on my iPhone 16 Pro Max, iios 18.5 and added the same wallpaper you show in your screenshot. As you can see in my screenshot there is no box as shown in yours. I do not have a widget on my Home Screen.

It's possible you have burn in on your iPhone screen. If you change your lock screen wallpaper, does that change what you see?