I recently implemented SSL pinning in my app to help protect against sniffing out API calls and found something interesting.
If you iOS project settings are set to 'HttpClient Implementation: Managed(default)' then you cant sniff the traffic whether it be HTTP or HTTPS using Charles, Burp or other common sniffers on a device or simulator.
Change the above setting to 'NSUrlSession' and then all of a sudden the traffic appears in the sniffers. I also noticed this same behavior on app store versions using these settings.
Useful for security purposes but took a while to figure out for debugging.
Food for thought.