It can be difficult to isolate an issue, which doesn't affect the vast majority of users. I have absolutely no issue making stickers on my iPhone 16 Pro Max. That doesn't mean you aren't having an issue, but when you ask why Apple hasn't fixed this yet, you appear to be assuming everyone is having the same issue you are. It's like walking into the hospital, seeing sick people and assuming everyone is sick.
You have a new iPhone, which came with a one year warranty. Perhaps it would make more sense instead of asking only other users on a user to user only forum, why questions, it would make sense to reach out to Apple Support directly for help. You can use the Support button at the bottom left of this page. If you're in the US, you can call Apple at: 800-MY-APPLE. You can visit your closest Apple Store Genius Bar.
And you can provide feedback to Apple here, so they are aware of your issue --> Feedback - iPhone - Apple