New iOS 18 iPhone Background and Lockscreen keeps changing back to the old one

After installing iOS 18 on my iPhone 12 mini, I also installed the new dynamic background (the blueish one) that came with the upgrade...

I noticed that it went back to my old iOS 17 version (the red/pink'ish one) every time I set the phone to the Do Not Disturb mode, and after I deactivate the Do Not Disturb mode, it stays on the old Background/Lockscreen.

I tried to change it many times back to the new one, both via the Lockscreen (when you can change the background without unlocking your phone), and then via the settings. But to no avail! It just goes back to the old screen. I think it's a bug specifically related to the iPhone and not the iPad... because it works fine on the iPad.

And I have a feeling that it has nothing to do with the fact that the background being dynamic.

Are there similar situations?