You may also use pyshark module for Python. You will find certificates in the SSL layer, but not all SSL layer packets have the certificate. Some of the packets are for handshake negotiation, etc. So, you can search for one of the x509 attributes in the packet.
# Import modules
# Docs: https://github.com/KimiNewt/pyshark/
I don't think that app is using http at all. I've seen similar behaviour with WhatsApp in which https is only used at the registration phase and then it switches to noise pipes which is not intercepted by http proxies.
A workaround is to use mitm_relay. It listens raw TCP/UDP packets, encapsulates them in http and forwards it to http proxy (burp). The only ...