I have the same problem.
I use a NEXUS 6P and even if my certificat is install I can't see the HTTPS traffic of my application. I can see HTTP traffic and Tunnel, I can contact the server and use the app normally but can't see HTTPS. I can see HTTPS of others applications on the device but not this one.
With an other device I don't have the problem. But I need to check HTTPS traffic in this specific device.
I'm an amateur in Fiddler and certificate so I prefer to ask.
(sorry my english is not ... good ^^")