I saw this tutorial, you can find, in the this post video, the button change background and foreground color when button state changed. but in the system UIBarButtonItem, and UIButton created by UIButton(type: .system) has a subtle transition when foreground color changed. I think this is better than no any transition. The new button api broke that. But in the UIBackgroundConfiguration document:
"When you apply a configuration to a view, UIKit performs the actual drawing and rendering of the background. When you use background configurations instead of rendering your own backgrounds, the system provides automatic view hierarchy management, support for interactive and interruptible animations and transitions, and performance optimizations."
So, I hope this is a bug. No transition is so bad.