@jimp:
That's just how IPsec works. It has no concept of routing, so you have to nudge the traffic to use the correct source:
https://doc.pfsense.org/index.php/Why_can%27t_I_query_SNMP,_use_syslog,_NTP,_or_other_services_initiated_by_the_firewall_itself_over_IPsec_VPN
Bingo. Add the static route with a /32 and now it's working perfectly. Thanks Jim!