Ran this script: https://people.freebsd.org/~rstone/scripts/subnet-route-test
On stock FreeBSD, the final "ifconfig" command fails with "ifconfig: ioctl (SIOCAIFADDR): File exists" and if you run "arp -na" afterwards you see a stray 2.0.0.1 that cannot be removed.
With this fix, the IP address can be assigned as expected.
I also tested changing a 2.0.0.0/8 route and verified that the IFA_ROUTE flag was not migrated because the subnet width was different.