I have fixed the issue. The icon size was incorrect. Once the size has been changed to the same for all layers, it should work. I also noticed that the layers were not separated for the first time. When I opened the app and closed the app, the layers got separated as expected.

Thanks for your reply. This information is very useful. However, I don't have much experience, but I will try my best to implement it. I have also filed a feedback request at https://feedbackassistant.apple.com/feedback/15404407 It would be great if there are some sample code to have a look. Thanks again.

After a lot of experiments, it started look like fire. Obviously, it is far from perfect. I hope I can help someone else as well. If there are better or more efficient approaches, I would like to know as well.