IPSEC vti adjacent /30 subnets cause routing problems with VPN traffic

nzkiwi68

pfSense 2.4.4-p3 & FRR 0.6.3_1

Site A - pfSense HA cluster SG-4860
Site B - pfSense HA cluster GX-1537

Site A, wan1 & wan2
Site B, wan1 & wan2

SiteA vti interfaces
wan1 vti 172.31.80.1 /30
wan2 vti 172.31.80.5 /30

SiteB vti interfaces
wan1 vti 172.31.80.2 /30
wan2 vti 172.31.80.6 /30

I want to prefer SiteA wan2 to SiteB wan2, so inside FRR under;
Services > FRR> OSPF> OSPF Interfaces
I simply weight the wan1 vti interface with cost 100 at both SiteA and SiteB end.

Job done, right?
What happens is lots of traffic gets lost, a ping from siteA to siteB mostly works, but is losing lots of traffic. There's absolutely nothing wrong with SiteA wan2 nor SiteB wan2.

In the end, all I did was this;

Make each of the vti interfaces /30 under a different /24.

SiteA vti interfaces
wan1 vti 172.31.81.1 /30
wan2 vti 172.31.82.1 /30

SiteB vti interfaces
wan1 vti 172.31.81.2 /30
wan2 vti 172.31.82.2 /30

I wonder if there's some obscure routing bug going on if you use two adjacent /30 subnets with vti interfaces?
Perhaps somewhere in the code it accidentally assumes /24 incorrectly.