Telnet minipot hangs after login prompt when conneted from WAN size

There seems to be some kind of issue with telnet minipot (or rather how it works with deployment). When user tries to connect to telnet minipot it hangs after login is entered (or requested). There seems to be no response given from minipot:

Trying xx.xx.xx.xx...
Connected to xx.xx.xx.xx.
Escape character is '^]'.
login: root

The weird thing is that the same connection from lan works (without redirect happening). It is also weird that telnet is deployed as any other minipot and those are not affected by this. It looks less like Firewall configuration issue and more like issue in telnet minipot that just surfaces because of firewall configuration.