взято отсюда https://docs.netgate.com/pfsense/en/latest/vpn/pptp-troubleshooting.html :
"pf does not have any capabilities of tracking more than one GRE connection per public IP per external host. That is, if the entire internal network gets NAT applied using the same public WAN IP, only one internal machine can connect to a given external GRE source. For PPTP, this means only one PC can connect to a given outside PPTP server at a time."
L2TP наверно тоже с GRE у вас