pfSense

interfaces.php checks for "This IPv4/IPv6 address conflicts with a Static Route" are too strict. Rather than only checking ip_in_subnet, it needs to check whether the IP is in the subnet and the subnet mask is exactly the same.

For instance, with a static route of 10.0.0.0/8, an interface with 10.0.0.1/24 configured as an IP is valid (but is currently rejected). A static route of 10.0.0.0/24 with an interface IP of 10.0.0.1/24 is not valid.

Same for IPv6.