OpenVPN won't route SOME traffics to port on SOME devices

[phucnb]

I have a set up:
- OpenVPN 2.5.9 and mitmproxy docker container running on port 8080.
I forward traffics from vpn to port by adding iptables rules
sudo iptables -t nat -A PREROUTING -i tun0 -p tcp --dport 80 -j REDIRECT --to-port 8080
sudo iptables -t nat -A PREROUTING -i tun0 -p tcp --dport 443 -j REDIRECT --to-port 8080
I have bunches of iOS devices installed openvpn client ant root CA from mitmproxy.
- When i use the proxy mitmproxy directly on all devices, I can see ALL the traffics from a specific app I want to inspect.
- When I use openvpn (which still redirect traffics to mitmproxy), I can see everything on SOME other devices while I don't see requests from the app I want on SOME devices but the requests still went throught (see responses on devices).
How can i trouble shooting? Is the problem from my vpn server, proxy (unlikely) or my devices?