I use a gateway group as the default gateway for both IPv4 and IPv6 and it works as expected - igb0 is tier 1 and igb1 is tier 2:
# netstat -rn | grep default default 192.168.1.254 UGS igb1 default fe80::da21:daff:fe19:dbb0%igb1 UG igb1 # ifconfig igb0 | grep status status: no carrierYou can share the files/logs here for review:
https://nc.netgate.com/nextcloud/s/Dj3ZbjQstNB52e7