if it was there from day one, yes, it could be assuming it's used for ADSL, and DHCP client only.
read WAN port does not work with ISP (BT Home Hub 5.0)
from that thread, it should look something like this in dmesg.
-- plugging WAN cable.
[10050.988789] lantiq,xrx200-net 1e108000.eth eth0: port 5 got link
[10050.994233] br-lan: port 1(eth0.1) entered blocking state
[10050.998843] br-lan: port 1(eth0.1) entered forwarding state
[10076.189174] nf_conntrack: default automatic helper assignment has been turned off for security reasons and CT-based firewall rule not found. Use the iptables CT target to attach helpers instead.
Seems it could be eth0.2 https://bugs.openwrt.org/index.php?do=details&task_id=490
BT Hub 5a Fresh Install, no internet