I just checked on pfSense, igc0 is not assigned but PPPoE is running over igc0 with the automagically adjusted MTU of 1508.
This is not shown in Status / Interfaces due to not being assigned, you have to check from SSH.
igc0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1508
options=e527bb<RXCSUM,TXCSUM,VLAN_MTU,VLAN_HWTAGGING,JUMBO_MTU,VLAN_HWCSUM,TSO4,TSO6,LRO,WOL_MAGIC,VLAN_HWFILTER,VLAN_HWTSO,RXCSUM_IPV6,TXCSUM_IPV6>
ether 00:e2:69:xx:xx:xx
inet6 fe80::2e2:69ff:fe59:25ee%igc0 prefixlen 64 scopeid 0x1
media: Ethernet autoselect (1000baseT <full-duplex>)
status: active
nd6 options=21<PERFORMNUD,AUTO_LINKLOCAL>
pppoe0: flags=88d1<UP,POINTOPOINT,RUNNING,NOARP,SIMPLEX,MULTICAST> metric 0 mtu 1500
description: WAN_ZEN
inet6 fe80::1%pppoe0 prefixlen 64 scopeid 0x11
inet6 2a02:8011:d000... prefixlen 64 autoconf
inet 82.69.xx.xx --> 51.148.xx.xx netmask 0xffffffff
nd6 options=23<PERFORMNUD,ACCEPT_RTADV,AUTO_LINKLOCAL>