UIImagePickerController not asking for permission

According to apple documentation

Note

When using the UIImagePickerController to bring up the user's photo library, your app doesn't need to request permission explicitly.

Photos automatically prompts the user to request authorization when needed.

The answer is simple: it's normal behavior since iOS 11, because the UIImagePickerController runs in a separate process and therefore for just read-only access you don't need any special permissions.