GRE Tunnel does not work if one of the endpoints is an IP Alias
See https://forum.pfsense.org/index.php?topic=88947.0 for more details.
I've reproduced this issue on two sets of equipment now.
Using the network map in the link above:
A GRE tunnel from 192.168.26.1 (IP Alias for R1) to 192.168.26.2 (WAN2 for R2) does not work.
If I change the IP endpoints to 192.168.25.1 (WAN for R1) to 192.168.25.1 (WAN for R2) it works fine.
Additionally when the GRE tunnel is attempting to use the IP Alias on R1 (Router with the IP Alias) it indicates the tunnel is down. I also see from packet captures, GRE pings from R2 to R1, but R1 never responds when it is using an IP Alias.