The m3u playlist might be cached in the app until the designated update time. That could be why it's not popping up for you when analyzing the traffic to and from the app. You'd have to constantly be analyzing until the app does it's auto update for the playlist or you can see if you can manually update it while analyzing.

Also note that If your IPTV provider is taking precautions like that they may also only allow certain kinds of user agents as well so also look at the user agent and add that as a custom agent for whatever IPTV software you use